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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.
.

an organizational unit or the function it performs.
Contemporary Examples

But there are also those to whom this act becomes not a transition for sex, but the center of activity.
Eight Strange Sex Fetishes Aurora Snow December 15, 2010

The activity is nearly exclusive to a bar setting, and the participants are normally intoxicated.
Dwarf Tossing Should Be Illegal Bill Klein October 22, 2011

The activity was so intense that there is now a severe shortage of fancy apartments for sale in New York.
December’s Economic Signs Looked Positive Amid Uncertainty Daniel Gross January 2, 2013

But the swirl of activity around her will reach its fevered pitch on November 24 when her lawyers launch her appeal.
Amanda Knox’s Other Trial Barbie Latza Nadeau September 30, 2010

The investigation stalled in 2008 over a wide range of questions about activity possibly related to developing nuclear weapons.
Exclusive: U.N. Won’t Back Down on Iran Nuclear Inspections Michael Adler March 10, 2012

Historical Examples

It enervates the powers of the mind, and benumbs the activity of man.
American Institutions and Their Influence Alexis de Tocqueville et al.

Early were hammers ringing on anvils in the Dragon Court, and all was activity.
The Armourer’s Prentices Charlotte M. Yonge

He was a short wrinkled old man, but appeared to possess great vigour and activity.
The Book of Enterprise and Adventure Anonymous

Its attributes of youth are the activity and eager life with which it is redundant.
Sketches from Memory Nathaniel Hawthorne

The activity and strength of the Frenchman were so great that a skilful pugilist would have found difficulty in handling him.
The Life of Kit Carson Edward S. Ellis

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.