spybug01A “bug” is a secret listening device that is used to listen in on conversations while not being in the same room.

It can be a tiny battery-powered microphone and transmitter (more easily detected, as it sends out a signal) or a recording device (often bigger, but since it doesn’t send out a signal so not so easily discovered. But you have to retrieve it later).

A bug is often placed inside a telephone, with the specific purpose of listening in on phone conversations.