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light-year

(Encyclopedia)light-year, in astronomy, unit of length equal to the distance light travels in one sidereal year. It is 9.461 × 1012 km (about 6 million million mi). Alpha Centauri and Proxima Centauri, the stars n...

mole, in chemistry

(Encyclopedia)mole, in chemistry, a quantity of particles of any type equal to Avogadro's number, or 6.02×1023 particles. One gram-molecular weight of any molecular substance contains exactly one mole of molecules...

barn

(Encyclopedia)barn, abbr. b, in physics, unit of nuclear cross section, i.e., the effective target presented by a nucleus for collisions leading to nuclear reactions; it is equal to 10−24 square centimeters. The ...

isobar

(Encyclopedia)isobar īˌsəbărˈĭk [key], line drawn on a weather map through points of equal atmospheric pressure. Isobars are used to define cyclones (low-pressure regions) and anticyclones (high-pressure regi...

angstrom

(Encyclopedia)angstrom ăngˈstrəm [key], abbr. Å, unit of length equal to 10−10 meter (0.0000000001 meter); it is used to measure the wavelengths of visible light and of other forms of electromagnetic radiatio...

STP

(Encyclopedia)STP or standard temperature and pressure, standard conditions for measurement of the properties of matter. The standard temperature is the freezing point of pure water, 0℃ or 273.15K. The standard...

equivalent weight

(Encyclopedia)equivalent weight. The equivalent weight of an element or radical is equal to its atomic weight or formula weight divided by the valence it assumes in compounds. The unit of equivalent weight is the a...

symmetry, biological

(Encyclopedia)symmetry, biological, similarity or balance between parts of an organism so that when a straight cut is made through a point or along a line, equal, mirror-image halves are formed. Symmetry in body sh...

Niagara Falls, waterfall, United States and Canada

(Encyclopedia)Niagara Falls, in the Niagara River, W N.Y. and S Ont., Canada; one of the most famous spectacles in North America. The falls are on the international line between the cities of Niagara Falls, N.Y., a...

Mechichi, Hichem

(Encyclopedia)Mechichi, Hichem, 1974–, Tunisian political leader. A lawyer by training, he served on the National Commission to Investigate Embezzlement and Corruption and with the National Anti-Corruption Author...