the time required for something to fall to half its initial value (in particular, the time for half the atoms in a radioactive substance to disintegrate)
a part of the cell containing DNA and RNA and responsible for growth and reproduction
any histologically identifiable mass of neural cell bodies in the brain or spinal cord
the positively charged dense center of an atom
(astronomy) the center of the head of a comet; consists of small solid particles of ice and frozen gas that vaporizes on approaching the sun to form the coma and tail
a single undivided whole; "an idea is not a unit that can be moved from one brain to another"
an organization regarded as part of a larger social group; "the coach said the offensive unit did a good job"; "after the battle the soldier had trouble rejoining his unit"
a single undivided natural thing occurring in the composition of something else; "units of nucleic acids"
an individual or group or structure or other entity regarded as a structural or functional constituent of a whole; "the reduced the number of units and installations"; "the word is a basic linguistic unit"
an administrative unit of government; "the Central Intelligence Agency"; "the Census Bureau"; "Office of Management and Budget"; "Tennessee Valley Authority"
the state of being in action or exerting power; "the agency of providence"; "she has free agency"