noun

• (uncountable) The state or quality of being active; activeness.
• (countable) Something done as an action or a movement.
• (countable) Something done for pleasure or entertainment, especially one involving movement or an excursion.
• (grammar, semantics) The lexical aspect (aktionsart) of verbs or predicates that change over time and have no natural end point.
• (physics) The number of radioactive decays per unit time. Unit for it: becquerel or curie
• The property of substances to react with other substances