Transponder
From Memory Alpha, the free Star Trek reference.
A transponder is a device on board a starship, starbase or other spacecraft, which broadcasts a signal identifying that craft by name, design and allegiance. This allows for easy recognition of craft especially at distances where visual inspection or accurate sensor readings are not possible.
Transponders can be deactivated or have their details forged, however, so the existence or absence of a transponder signal is not proof-positive of a craft's identity or existence. For this reason transponder signals can include some kind of code proving to those 'in-the-know' that the signal is genuine.
