ˈæk.ʃən ækˈtɪ.vɪ.ti any specific behavior they avoided all recreational activity the trait of being active; moving or acting rapidly and energetically the level of activity declines with age an organic process that takes place in the body respiratory activity a process existing in or produced by nature (rather than by the intent of human beings) the action of natural forces volcanic activity the state of being active his sphere of activity he is out of action (chemistry) the capacity of a substance to take part in a chemical reaction catalytic activity