An action is something which is done by an agent. In common speech, the term action is often used interchangeably with the term behavior. In the philosophy of action, the behavioural sciences, and the social sciences, however, a distinction is made: behavior is defined as automatic and reflexive activity, while action is defined as intentional, purposive, conscious and subjectively meaningful activity . Thus, throwing a ball is an instance of action; it involves an intention, a goal, and a bodily movement guided by the agent. On the other hand, catching a cold is not considered an action because it is something which happens to a person, not something done by one.

Action theory is an area in philosophy concerned with theories about the processes causing willful human bodily movements of a more or less complex kind. This area of thought has attracted the strong interest of philosophers ever since Aristotle's Nicomachean Ethics (Third Book). With the advent of psychology and later neuroscience, many theories of action are now subject to empirical testing.