One that acts or has the power or authority to act. One empowered to act for or represent another: an author's agent; an insurance agent. A means by which something is done or caused; instrument. A force or substance that causes a change: a chemical agent; an infectious agent. A representative or official of a government or administrative department of a government: an FBI agent. A spy. Linguistics The noun or noun phrase that specifies the person through whom or the means by which an action is effected. To act as an agent or representative for: Who will agent your next book? To act as an agent or representative.