In linguistics, Agent (grammar), one of the thematic roles: the participant of a situation that carries out an action. Replaces "subject" in placement typology.
Agent (law), a person authorised to act on behalf of another person
Agent provocateur, A person assigned to provoke unrest, violence, debate, or argument by or within a group while acting as a member of the group but covertly representing the interests of another
Agent noun (or nomen agentis), one of the derivatemes (a topic from linguistic morphology)