n.

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