S: (n) activity ((chemistry) the capacity of a substance to take part in a chemical reaction) "catalytic activity"
S: (n) natural process, natural action, action, activity (a process existing in or produced by nature (rather than by the intent of human beings)) "the action of natural forces"; "volcanic activity"
S: (n) activeness, activity (the trait of being active; moving or acting rapidly and energetically) "the level of activity declines with age"