In philosophy, an action is that which is done by an agent. In common speech, the term is often used interchangeably with the term "behaviour". However, in the philosophy of action, behavioural sciences, and the social sciences, a distinction is made: behavior is automatic and reflexive activity, while action is an intentional, purposive, conscious and subjectively meaningful activity. Thus, things like running or throwing a ball is an instance of action; it involves 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.
In enactivism theory, perception is understood to be sensorimotor in nature. That is, we carry out actions as an essential part of perceiving the world. Alva Noë states: 'We move our eyes, head and body in taking in what is around us... [we]: crane our necks, peer, squint, reach for our glasses or draw near to get a better look...'...'Perception is a mode of activity on the part of the whole animal...It cannot be represented in terms of merely passive, and internal, processes...' 
Some would prefer to define actions as requiring bodily movement (see behaviorism). The side effects of actions are considered by some to be part of the action; in an example from Anscombe's manuscript Intention, pumping water can also be an instance of poisoning the inhabitants. This introduces a moral dimension to the discussion (see also Moral agency). If the poisoned water resulted in a death, that death might be considered part of the action of the agent that pumped the water. Whether a side effect is considered part of an action is especially unclear in cases in which the agent isn't aware of the possible side effects. For example, an agent that accidentally cures a person by administering a poison he was intending to kill him with.
A primary concern of philosophy of action is to analyze the nature of actions and distinguish them from similar phenomena. Other concerns include individuating actions, explaining the relationship between actions and their effects, explaining how an action is related to the beliefs and desires which cause and/or justify it (see practical reason), as well as examining the nature of agency. A primary concern is the nature of free will and whether actions are determined by the mental states that precede them (see determinism). Some philosophers (e.g. Donald Davidson) have argued that the mental states the agent invokes as justifying his action are physical states that cause the action. Problems have been raised for this view because the mental states seem to be reduced to mere physical causes. Their mental properties don't seem to be doing any work. If the reasons an agent cites as justifying his action, however, are not the cause of the action, they must explain the action in some other way or be causally impotent.