Definition of "activity"

(uncountable) The state or quality of being active; activeness.  examples

(countable) Something done as an action or a movement.  examples

(countable) Something done for pleasure or entertainment, especially one involving movement or an excursion.  quotations  examples

(grammar, semantics) The lexical aspect (aktionsart) of verbs or predicates that change over time and have no natural end point.  quotations  examples

(physics) The number of radioactive decays per unit time. Unit for it: becquerel or curie  examples

The property of substances to react with other substances  examples