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the state or quality of being active:
There was not much activity in the stock market today. He doesn’t have enough physical activity in his life.
a specific deed, action, function, or sphere of action:
social activities.
work, especially in elementary grades at school, that involves direct experience by the student rather than textbook study.
energetic activity; animation; liveliness.
a use of energy or force; an active movement or operation.
normal mental or bodily power, function, or process.
Physical Chemistry. the capacity of a substance to react, corrected for the loss of reactivity due to the interaction of its constituents.
Physics.

the number of atoms of a radioactive substance that disintegrate per unit of time, usually expressed in curies.
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an organizational unit or the function it performs.
noun (pl) -ties
the state or quality of being active
lively action or movement
any specific deed, action, pursuit, etc: recreational activities
the number of disintegrations of a radioactive substance in a given unit of time, usually expressed in curies or disintegrations per second

the capacity of a substance to undergo chemical change
the effective concentration of a substance in a chemical system. The absolute activity of a substance B, λB, is defined as exp (μBRT) where μB is the chemical potential

n.

c.1400, “active or secular life,” from Old French activité, from Medieval Latin activitatem (nominative activitas), a word in Scholastic philosophy, from Latin activus (see active). Meaning “state of being active, briskness, liveliness” recorded from 1520s; that of “capacity for acting on matter” is from 1540s.

activity ac·tiv·i·ty (āk-tĭv’ĭ-tē)
n.

A physiological process.

The presence of neurogenic electrical energy in electroencephalography..

An ideal concentration for which the law of mass action will apply perfectly.

The intensity of a radioactive source.

The ability to take part in a chemical reaction.