To wiretap is to secretly listen in on someone's phone conversation. In some cases, the FBI will use a wiretap to investigate a serious federal crime.
A wiretap is either the act of surveillance with a listening device or the device itself: "They put a wiretap on the mob boss's home phone, but he always uses a cellphone." As a verb, wiretap means to use such a device: "The government can't wiretap someone without a warrant." Since the invention of the telephone, law enforcement has used wiretaps — originally from the sense of tapping (obtaining information from) a telephone wire.