Definition of activity:
any specific behavior; "they avoided all recreational activity"
the state of being active; "his sphere of activity"; "he is out of action"
an organic process that takes place in the body; "respiratory activity"
(chemistry) the capacity of a substance to take part in a chemical reaction; "catalytic 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 trait of being active; moving or acting rapidly and energetically; "the level of activity declines with age"