An eavesdropper is someone who listens in on a conversation that they are not supposed to be a part of. Other synonyms for an eavesdropper include a listener, a spy, a snoop, a voyeur, a peeping Tom, a nosy parker, and an intruder. The term "listener" is a simple and direct way to describe someone who is listening in on a conversation. A spy and a snoop may be used in the context of someone who is intentionally gathering information that they should not have access to. A voyeur and a peeping Tom typically refer to someone who is spying on others in a more secluded or private setting. A nosy parker and an intruder both suggest that the person is being intrusive in some way.