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atoll a ring-shaped island or reef that surrounds a lagoon.
atom an invisible component of any element, consisting of protons, neutrons, and electrons, once considered the smallest possible unit of matter. [2 definitions]
atom bomb see atomic bomb.
atomic of, relating to, or using atoms or energy generated by atoms. [3 definitions]
atomic age (often cap.) the period in history characterized by the development and use of atomic energy in the generation of electrical power and for immensely destructive weapons.
atomic bomb an extremely destructive weapon, the explosive power of which derives from the immense amount of energy suddenly released when the nuclei of uranium 235 or plutonium atoms are split; atom bomb.
atomic clock an extremely accurate timekeeping device regulated by the reliably constant frequency at which atoms of certain highly reactive substances absorb or emit electromagnetic radiation.
atomic cocktail (informal) a dose of a liquid, taken orally, that contains a radioactive substance used in the diagnosis or treatment of cancer.
atomic energy the energy that is released by an atom in nuclear reactions such as fission or fusion. [2 definitions]
atomicity the number of atoms in a molecule. [2 definitions]
atomic mass unit a unit equal to one twelfth of the mass of an atom of the most abundant kind of carbon, equal to 1.6605 times 1024 gram.
atomic number the number of protons or positive electrical charges in the nucleus of one atom of a given element. [2 definitions]
atomic pile see nuclear reactor.
atomic theory any of several theories concerning the structure, properties, or behavior of atoms. [2 definitions]
atomic weight the average weight of one atom of a given element, expressed in atomic mass units.
atomize to divide into atoms. [3 definitions]
atomizer a device that converts a liquid, esp. a medicine or perfume, into a spray.
atom smasher see accelerator.
atonal in music, composed without a fixed key or scale.
at once at the same time. [2 definitions]
atone to make reparation for a sin, crime, error, or the like (usu. fol. by for). [2 definitions]