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The American HeritageŽ Student Science Dictionary

S - Z

sex-linked Carried on or transmitted by a sex chromosome. Sex-linked inheritance concerns the genes located on the sex chromosomes. Color blindness in humans is a sex-linked trait.

space-time The four dimensions in which all objects are located and all events occur, viewed as a single and continuous framework for existence. Space-time consists of length, width, and depth, plus the dimension of time. See more at relativity.

Did You Know?
Before Albert Einstein's theory of relativity, people thought they lived in a three-dimensional world. Time was considered to be a completely separate entity. But Einstein realized that a more correct view put time and the three space dimensions on equal footing. Physicists call the four-dimensional construct within which they interpret nature space-time. In space-time two events that appear to happen at the exact same time to one person in one place can appear to happen at different times to someone else in a different place. In other words, whether or not the events occur at the same time depends on the position and movement of the observers. How is this possible? Think of thunder and lightning. Lightning reaches the eye at the speed of light, while thunder reaches the ear at the much slower speed of sound. To an observer right where lightning hits, the thunder seems to come at the same moment. Several miles away, however, the lightning appears well before the thunder is heard. Similarly, if one observer were moving faster than the other toward or away from the lightning strike, each observer would experience the events at different time intervals. As Einstein figured it, everything in space-time is relative.


sto·ma 1. Botany. One of the tiny openings in the outer surface of a plant leaf or stem, through which gases and water vapor pass. Most stomata are on the underside of leaves. 2. Zoology A mouth-like opening, such as the oral cavity of a nematode.
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strong nuclear force A force between elementary or subatomic particles that holds quarks together to form protons and neutrons, and also binds protons and neutrons together in the atomic nucleus. The strong nuclear force is one of the four basic forces in nature, along with gravity, the electromagnetic force, and the weak nuclear force. Also called strong interaction.

torque The tendency of a force applied to an object to make it rotate about an axis. Torque is equal to the amount of the force acting on the object multiplied by the distance from its point of application to the axis around which the object rotates (or would rotate if it were not fixed in place).
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tran·scrip·tion The process in a cell by which genetic material is copied from a strand of DNA to a complementary strand of RNA (called messenger RNA). Transcription takes place in the nucleus before messenger RNA is transported to the ribosomes, the places in the cell where proteins are made.
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trans·form·er ) A device used to change the voltage of an alternating current in one circuit to a different voltage in a second circuit. Transformers consist of a frame-like iron core that has a wire wound around each end. As a current enters the transformer through one of the coils, the magnetic field it produces causes the other coil to pick up the current. If there are more turns on the second coil than on the first coil, the outgoing current will have a higher voltage than the incoming current. This is called a step-up transformer. If there are fewer turns on the second coil than on the first, the outgoing current will have a lower voltage. This is called a step-down transformer.

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vac·uum 1. A condition in which there is no matter or very little matter. 2. An enclosed space, such as the space inside a container, in which there are far fewer gas molecules than in an equal volume of the air outside it. A vacuum has a much lower gas pressure than that of the atmosphere at sea level.
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va·lence A whole number that represents the ability of an atom or a group of atoms to combine with other atoms or groups of atoms. The valence is determined by the number of electrons that an atom can lose, add, or share. A carbon atom, for example, can share four electrons with other atoms and therefore has a valence of 4.
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vec·tor 1. A quantity, such as velocity or change of position, that has both magnitude and direction. Compare scalar. 2. An organism, such as a mosquito or tick, that spreads disease-causing microorganisms from one host to another without harm to itself.
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ver·te·brate Any of a large group of animals having a backbone, including fish, amphibians, reptiles, birds, and mammals. Vertebrates are bilaterally symmetrical and have an internal skeleton of bone or cartilage, a nervous system along the back that is divided into brain and spinal cord, and not more than two pairs of limbs.
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vis·cos·i·ty The resistance of a substance to flow. A substance that can flow easily has a low viscosity. A substance that cannot flow easily has a high viscosity.
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water cycle The continuous process by which water is distributed throughout the Earth and its atmosphere. Energy from the sun causes water to evaporate from oceans and other bodies of water and from soil surfaces. Plants and animals also add water vapor to the air by transpiration. As it rises into the atmosphere, the water vapor condenses to form clouds. Rain and other forms of precipitation return water to the Earth, where it flows into bodies of water and into the ground, beginning the cycle over again. Also called hydrologic cycle.


wave·length The distance between one peak or crest of a wave and the next peak or crest.
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xy·lem A tissue in vascular plants that carries water and dissolved minerals up from the roots through the stem to the leaves and provides support for the softer tissues. Xylem consists of various elongated cells that function as tubes. In a tree trunk, the innermost part of the wood is dead but structurally strong xylem, while the outer part consists of living xylem, and beyond it, layers of cambium and phloem. See more at cambium, photosynthesis. Compare phloem.

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zy·gote The cell formed by the union of the nuclei of two reproductive cells (called gametes), especially a fertilized egg cell.
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Some of the words on this list include cross-references to entries in the dictionary that are not included in the list itself.




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