1. a person or entity that acts or has the capacity to act, particularly on behalf of another or of a group. For example, an agent may be a therapist who helps a client gain self-understanding or an individual who initiates a social interaction.

2. a means by which something is done or caused. For example, an infectious agent is a bacterium or other microorganism that causes a particular disease.

3. in linguistics, the entity that performs the main action in a clause or sentence. It is prototypically an animate noun and usually, but not always, the grammatical subject of the clause. The agent is an important category in case grammar. Compare experiencer; instrumental; patient.

4. in parapsychology, the person who instigates an alleged occurrence of psychokinesis or telepathy. Compare percipient. See also sender.

5. in philosophy, an entity that possesses both the power to bring about an event and the power to refrain from bringing it about. In this sense, an agent possesses free will or some similar power to act or forbear. A person as agent is considered to be partly or wholly the originator of his or her own actions.

6. a computer program that makes use of distributed cognition in problem solving. This type of program is designed for use in such areas as telecommunications and Internet-based tasks. —agentive adj.