Someone who provides you with information. Anyone who has ever talked to an intelligence agent is considered a source. An agent who says, “I have a source in the White House,” can mean anything from having once been told to move on by the guard at the gate to screwing the President’s wife. Source is an interesting but meaningless phrase. Reliable source is nicer. Always ask the agent why he thinks the source is reliable. This will give you time to read your newspaper while he thinks up an answer.