ac·tiv·i·ty (?k-t?v??-t?)n. pl. ac·tiv·i·ties 1. The state of being active.2. Energetic action or movement; liveliness.3. a. A specified pursuit in which a person partakes.b. An educational process or procedure intended to stimulate learning through actual experience.4. The intensity of a radioactive source.5. The ability to take part in a chemical reaction.6. A physiological process: respiratory activity.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.activity (æk?t?v?t?) n, pl -ties1. the state or quality of being active2. lively action or movement3. any specific deed, action, pursuit, etc: recreational activities. 4. (General Physics) the number of disintegrations of a radioactive substance in a given unit of time, usually expressed in curies or disintegrations per second5. (Chemistry) a. the capacity of a substance to undergo chemical changeb. the effective concentration of a substance in a chemical system. The absolute activity of a substance B, ?B, is defined as exp (?BRT) where ?B is the chemical potentialCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014ac?tiv?i?ty (æk?t?v ? ti) n., pl. -ties. 1. the state or quality of being active. 2. energetic activity; animation; liveliness. 3. a specific deed, action, function, or sphere of action: social activities. 4. an educational task that involves direct experience and participation of the student. 5. a use of energy or force; an active movement or operation. 6. normal mental or bodily power, function, or process. 7. the capacity of a chemical substance to react, corrected for the loss of reactivity due to the interaction of its constituents. 8. a. the number of atoms of a radioactive substance that disintegrate per unit of time, usu. expressed in curies. b. radioactivity. 9. an organizational unit or the function it performs. [1520?30; (