From Hull AWE
In traditional grammatical terminology, an agent, or agent-noun, is
- in function, the 'doer', or 'performer, of an action. It is essentially the same thing as the Subject of a verb. In this sense of 'agent', there is a corollary word for the Object, 'the receiver of the action' - the patient. This is not in common use nowadays.
- In form, an agent-word is a noun, usually formed directly from a verb, which denotes the 'person who does' the action denoted by the verb. Thus a 'doer' is the person who 'does', an 'opener' is a person (or thing) that 'opens', and a 'keeper' is a 'person (or thing) that keeps'.