Houghton Mifflin Trade and Reference DivisionHoughton MifflinHoughton Mifflin Trade and Reference Division

Detailed Search


Press Release


The American HeritageŽ Student Science Dictionary

A - F

ac·cel·er·a·tion The rate of change of the speed or direction of a moving body with respect to time. See more at gravity, relativity.

Did You Know?
Most people know that an object has weight because of the pull of gravity, but did you know that weight is actually an indication that an object is being accelerated? When you're in an elevator, for example, as the elevator accelerates upward or downward you feel as if your weight is changing—you feel heavier when the elevator is accelerating upward, and lighter as it accelerates downward. Stand on a bathroom scale in the elevator, and you'll see that the effect is real: the readout on the scale does indeed change as the elevator accelerates. When it accelerates upward and you feel heavier, the readout increases; when it accelerates downward and you feel lighter, the readout decreases. Exactly what is changing as you move upward and downward in the elevator? It isn't your mass—the amount of matter in your body. That remains the same. Actually, it's your acceleration that is changing. Your speed and direction are changing, as the elevator moves faster or slower and goes up or down. So the changes in your weight shown on the scale actually are a measure of changes in your acceleration.

Listen to Pronunciation

a·mi·no acid Any of a large number of compounds that are found in living cells, contain carbon, oxygen, hydrogen, and nitrogen, and join together to form proteins. About 20 amino acids are needed by animal cells to produce proteins, but only about half, called nonessential amino acids, can be produced by animal cells. The remaining half, called essential amino acids, must be obtained from food.
Listen to Pronunciation

an·gi·o·sperm Any of a large group of plants that have flowers and produce seeds enclosed in an ovary or a fruit; a flowering plant. Most living plants are angiosperms. Compare gymnosperm.
Listen to Pronunciation

an·thrax An infectious, usually fatal disease of mammals, especially cattle and sheep, caused by a bacterium. It can spread to people, causing symptoms ranging from blistering of the skin to potentially fatal infection of the lungs.
Listen to Pronunciation

aus·tra·lo·pith·e·cine Any of several early hominids of eastern and southern Africa, known from fossils dating from about four million to about one million years ago. The most complete australopithecine skeleton found so far, named Lucy by its discoverers, is estimated to be just over three million years old. While many scientists believe that australopithecines are ancestors of modern humans, not enough fossils have yet been found to establish any direct descent.
Listen to Pronunciation

Avogadro's number The number of atoms or molecules in a mole of a substance, approximately 6.0225 × 1023. See more at mole3.
Listen to Pronunciation

big bang The violent explosion of an extremely small, hot, and dense body of matter between 12 and 18 billion years ago. It is viewed as the earliest event in a widely held model of the origin of the universe. Compare steady state universe.

Did You Know?
It's a chilling thought: In the 1920s, astronomers found that wherever they looked in space, distant galaxies were rapidly moving away from Earth. In other words, the universe was getting larger and larger. By calculating the speed of several galaxies and working back from there, astronomers learned that this expansion began between 12 and 18 billion years ago, when the entire universe was smaller than a dime and almost infinitely dense. According to the widely accepted theory of the big bang, a massive explosion kicked off the expansion and was the origin of space and time. Now scientists must figure out how much mass the universe contains in order to see what lies ahead. If there is enough mass, the gravity attracting all the pieces to each other will eventually stop the expansion and pull all the pieces of the universe back together in a "big crunch." The universe would then be a closed universe. However, there may not be enough mass to support a universe that is closed. If that is the case, then an open universe would expand forever, and all the galaxies and stars would drift away from each other and become dark and cold.


bi·o·di·ver·si·ty The number and variety of different organisms found within a specified geographic region.
Listen to Pronunciation

bi·ome A large community of plants and animals that occupies a distinct region defined by its climate and dominant vegetation. Grassland, tundra, desert, tropical rain forest, and deciduous and coniferous forests are all examples of biomes.

Listen to Pronunciation

black hole An extremely dense celestial object that has a gravitational field so strong that nothing can escape, not even light. A black hole is formed by the collapse of a massive star's core in a supernova. See more at star.

Did You Know?
One of the strangest objects in the universe is the burnt-out remnant of a large star, known as a black hole. The name comes from the fact that the star collapses into itself, becoming so dense that its gravitational pull keeps even light from escaping. And if light can't get out, then nothing that ever enters the black hole would ever escape. Rockets to the moon or Mars need to achieve what is called escape velocity, the speed necessary to overcome the Earth's gravity. But since nothing can ever go faster than the speed of light, nothing could ever go fast enough to reach the escape velocity necessary to pull out of a black hole. Here's how dense a black hole is: the sun has a diameter of about 864,000 miles (1,391,000 kilometers); for it to be as dense as a black hole, its entire mass would have to be squeezed down to a ball less than two miles across.


Brown·i·an motion The random movement of microscopic particles suspended in a liquid or gas, caused by collisions between these particles and the molecules of the liquid or gas.
Listen to Pronunciation

cat·a·lyst A substance that starts or speeds up a chemical reaction while undergoing no permanent change itself. The enzymes in saliva, for example, are catalysts in digestion. —catalyze verb
Listen to Pronunciation

cen·trip·e·tal force The force that pulls an object moving in a circle toward the center of the circle and causes the object to follow a curving path. Earth's gravity acts as a centripetal force on the moon.

Did You Know?
In one popular carnival ride, people stand with their backs against the wall of a cylindrical chamber. The chamber spins rapidly and then the floor drops out, but the riders remain pressed against the wall and don't fall down. Why? Most people would say that the reason people "stick" to the wall is because a centrifugal, or outward, force is pushing them against it. In actuality, there is no outward force, no matter how strongly the people on the ride may think they feel one. In fact, it's just the opposite: the riders are really subject to an inward, or centripetal, force. As the ride spins, it forces the riders to travel in a circle. Objects (including people) in motion tend to travel in a straight line at constant speed unless they're acted on by some external force. To make an object travel along a curved path, you have to keep forcing it toward the "inside" of the curve. The walls of the ride do just that, pushing the riders toward the center; the friction between the riders and the wall holds them up, so they seem to defy gravity.

Listen to Pronunciation

CFC Abbreviation of chlorofluorocarbon. See under fluorocarbon.

clone Noun 1. A cell, group of cells, or organism that is produced asexually from a single ancestor. The cells of an individual plant or animal are clones because they all descend from a single fertilized cell. A clone may be produced by fission, in the case of single-celled organisms, or by budding, as in the hydra. Some plants can produce clones from horizontal stems, such as runners. Clones of cells and some plants and animals can also be produced in a laboratory. 2. A copy of a sequence of DNA, as from a gene, that is produced by genetic engineering. The clone is then transplanted into the nucleus of a cell from which genetic material has been removed. Verb 1. To produce or grow a cell, group of cells, or organism from a single original cell. 2. To make identical copies of a DNA sequence. See more at genetic engineering.
Listen to Pronunciation

con·duc·tion The flow of energy, such as heat or an electric charge, through a substance. In heat conduction, the energy flows by direct contact of the substance's molecules with each other. Although the molecules vibrate, they do not change position in the transfer of energy. In electrical conduction, energy flows by the movement of electrons or ions.

Did You Know?
Heat is a form of energy that results in the motion of molecules. Heat travels by conduction, convection, or radiation. In conduction, heat spreads through a solid by making its molecules vibrate faster. As faster molecules bump slower ones, the slower ones are made to vibrate faster, and the solid becomes hotter. This is how the handle of a teaspoon sticking out of a cup of hot tea eventually gets hot. When liquids and gases are heated, their molecules, which are free to move about, move farther apart. The hotter portions of the liquid or gas expand, become less dense, and rise, and cooler portions move down to take their place. This movement causes the liquid or gas to circulate in the process called convection. The currents of the ocean are convection currents caused by the uneven heating of the ocean waters by the sun. Radiation carries heat in the form of waves through space. A hot object, like the hot wire in a heat lamp, gives off energy waves called infrared rays. When these rays strike an object, its molecules absorb the rays' energy and vibrate or move faster, and so the object becomes hotter. The sunlight that warms your face has traveled through 93 million miles of space by radiation.

Listen to Pronunciation

con·vec·tion The transfer of heat energy through liquids and gases by the movement of molecules. When molecules of the liquid or gas come in contact with a source of heat, they move apart and away from the source of heat, and cooler molecules take their place. Eventually, as the cooler molecules are heated, they move as well, and a convection current forms, transferring the heat. See Note at conduction.
Listen to Pronunciation

cy·to·plasm The jelly-like material that makes up much of a cell inside the cell membrane, and, in eukaryotic cells, surrounds the nucleus. The organelles of the cell, such as mitochondria and (in green plants) chloroplasts, are contained in the cytoplasm. The cytoplasm together with the nucleus make up the cell's protoplasm. See more at cell.
Listen to Pronunciation

DNA Short for deoxyribonucleic acid. The nucleic acid that is the genetic material determining the makeup of all living cells and many viruses. It consists of two strands of nucleotides linked together in a structure resembling a ladder twisted into a spiral. In eukaryotic cells, the DNA is contained mainly in the nucleus and mitochondria. DNA can replicate itself and synthesize RNA. Compare RNA. See Note at gene.

Did You Know?
One of the wonders of nature is that the complexity and diversity of life can be contained in a molecule with a relatively simple structure. Deoxyribonucleic acid, commonly called DNA, exists mainly in the nucleus and mitochondria of each cell in an organism. It consists of two long strands linked together in a structure resembling a ladder twisted into a spiral, called a double helix. Each rung is made up of two chemical bases, called nucleotides, that are joined together by hydrogen bonds. There are four kinds of nucleotides in a DNA molecule: cytosine, guanine, adenine, and thymine—C, G, A, and T, for short. Specific sequences of these bases, known as genes, form codes that contain all of an organism's genetic information. When other components of a cell "read" this code, they produce proteins, the building blocks of life.

Listen to Pronunciation

Dop·pler effect The apparent change in the frequency of waves, as of sound or light, when the source of the waves is moving toward or away from an observer.


Did You Know?
When a car rushes past you on the road with the driver holding down the horn, you hear the horn change tone: it's higher pitched than normal as the car approaches and lower pitched as it departs. That's because of the Doppler effect. Sound waves spread outward in all directions from the horn. The forward motion of the car compresses the sound waves traveling ahead of the car, making the wavelengths shorter. Sound having shorter wavelengths has higher frequency and therefore higher pitch—what you hear if the car is moving towards you. Behind the car, however, the sound waves are drawn apart. Longer wavelengths mean lower frequency and lower pitch, which is what you hear once the car rushes past. The Doppler effect works on light waves, too; in fact, this was how scientists determined that the universe is expanding. The light from galaxies and other distant celestial objects is shifted toward the red end of the spectrum (a phenomenon called red shift). Red light has the longest wavelengths of visible light. The pioneering astronomer Edwin Hubble reasoned that the red shift was due to the Doppler effect: the galaxies are speeding away from us, drawing out the wavelengths of the light emitted behind them, and the universe as a whole is expanding.

Listen to Pronunciation

ec·o·sys·tem An ecological community made up of plants, animals, and microorganisms together with their environment. A pond or a rain forest are each examples of complex ecosystems.
Listen to Pronunciation

electromagnetic spectrum The entire range of electromagnetic radiation. At one end of the spectrum are gamma rays, which have the shortest wavelengths and high frequencies. At the other end are radio waves, which have the longest wavelengths and low frequencies. Visible light, with intermediate wavelengths and frequencies, is near the center of the spectrum.

Listen to Pronunciation

El Ni·ño A warming of the surface water of the eastern and central Pacific Ocean, occurring every 4 to 12 years and causing unusual weather patterns. The warmer water kills fish and plankton, brings heavy rains to western South America, and causes drought in eastern Australia and Indonesia. Compare La Niña.
Listen to Pronunciation

e·qui·nox 1. Either of the two moments of the year when the sun crosses the celestial equator. The vernal equinox occurs on March 20 or 21, and the autumnal equinox occurs on September 22 or 23. The days on which an equinox falls have about equal periods of sunlight and darkness. 2. Either of the two points on the celestial sphere where the apparent path of the sun (known as the ecliptic) crosses the celestial equator. Compare solstice.

Listen to Pronunciation

eu·kar·y·ote An organism whose cells contain a nucleus surrounded by a membrane. All organisms except for bacteria, cyanobacteria, and the bacteria-like organisms known as archaea are eukaryotes. Compare prokaryote.
Listen to Pronunciation

fault A crack in a rock mass along which there has been movement. The rock on one side of the crack moves relative to the rock on the other side of the crack. Faults are caused by plate-tectonic forces. See Note at earthquake.


Did You Know?
Bedrock is often cracked along surfaces known as planes. In some places the cracks extend only a tiny distance; in others they can run for hundreds of miles. When the rocks separated by a crack move past each other, the cracks are known as faults. The rocks move because they are pushed or pulled by the forces of plate tectonics. This movement often occurs in sudden jerks known as earthquakes. Geologists study faults to learn the history of the forces that have acted on rocks. Normal faults occur when rocks are being pulled apart. In this case, the rocks above the fault plane are moving down relative to the rocks below it. When rocks are pushed together, the opposite happens—the rocks above the plane move upward relative to the rocks below the plane; these types of faults are called reverse faults. Strike-slip faults occur when rocks slide past each other; rocks on either side of the crack slide parallel to the fault plane between them. Transform faults are a special category of strike-slip faults in which the crack is actually part of a boundary between two enormous tectonic plates. This is the nature of the famous San Andreas Fault in California.

Listen to Pronunciation

fiber optics The technology based on the use of fine glass or plastic fibers that are capable of transmitting light around curves. Fiber optics is used in medicine and for long-distance telephone and computer lines.

Did You Know?
In an optical fiber, a beam of light travels within a thin strand of glass or plastic. The light stays within the strand, even if the strand is curved or twisted. That's because the materials the optical fiber is made of—and the way in which the light is aimed into the fiber—are chosen so that when the light beam reaches the strand's outer edge it reflects back into the strand, rather than escaping through the wall. Generally, when a beam of light traveling in one material strikes the boundary of another, some light travels through the boundary and some is reflected back into the original material. But if the speed of light in the material the beam starts out in (the glass) is lower than the speed of light in the other material (the air or insulating material surrounding the fiber), and if the light strikes the boundary of the other material at a shallow enough angle, then all of the light is reflected and none escapes.


fis·sion 1. The splitting of the nucleus of an atom into two or more nuclei. The splitting occurs either spontaneously, because the nucleus has many neutrons and is unstable, or because the nucleus has collided with a free-moving neutron. The splitting of a nucleus releases one or more neutrons and energy in the form of radiation. Compare fusion. 2. A reproductive process in which a single cell splits to form two independent cells that later grow to full size. Bacteria and other single-celled organisms usually reproduce by means of fission. Also called binary fission.
Listen to Pronunciation

force 1. Something that causes a body to move, changes its speed or direction, or distorts its shape. One force may be counteracted by another, so that there is no change or distortion. 2. Any of the four natural phenomena exerting an influence between particles of matter. From the strongest to the weakest, these four forces are the strong nuclear force, the electromagnetic force, the weak nuclear force, and gravity.

Did You Know?
The verb force might make you think of pushing really hard on a stuck door or of banging the bottom of a stubborn ketchup bottle. The scientific meaning of the noun force also involves getting an object to move. In the mid-1600s, the great English physicist Isaac Newton figured out that the amount of force needed to move an object was directly related to both the mass of the object and how it is accelerated. (Pushing a pebble clearly takes less force than pushing a boulder, and pushing a boulder quickly obviously takes more force than pushing it slowly.) What is now known as Newton's second law of motion sets down this relationship quantitatively: Force equals mass times acceleration, or F = ma. You see this equation in action every time you step on a scale. Your weight is actually the downward force that results from your body mass being pulled—accelerated—by gravity. Remember that acceleration here means a change in direction or in speed, either faster or slower. A boat that bumps a dock comes momentarily to a standstill. That rapid decrease in speed multiplied by the mass of the boat is the force with which the boat hits the dock.

Listen to Pronunciation

frac·tal A geometric pattern repeated at ever smaller scales to produce irregular shapes and surfaces that cannot be represented by standard geometry. Even the most minute details of a fractal's pattern repeat elements of the overall geometric pattern. Fractals are widely used in computer modeling of irregular patterns and structures in nature, such as the patterns of seasonal weather. They are also considered to be a visual representation of chaos. See more at chaos.
Listen to Pronunciation

fu·sion 1. The joining together of light atomic nuclei, especially hydrogen nuclei, to form a heavier nucleus, especially a helium nucleus. Fusion occurs when light nuclei are heated to extremely high temperatures, forcing them to collide at great speed. The collision releases one or more neutrons and energy in the form of radiation. Fusion reactions power the sun and other stars. See more at fission. 2. A mixture or blend formed by fusing two or more things: An alloy is a fusion of two or more metals.
Listen to Pronunciation




Some of the words on this list include cross-references to entries in the dictionary that are not included in the list itself.




Booksellers Home | Trade Home | FAQ | Site Map
Privacy Policy | Trademark Information | Terms and Conditions of Use
Copyright © Houghton Mifflin Company. All rights reserved.