dongle

A portable, physical, usually electronic device that is required to be attached to a computer to enable a particular software program to run. (See: token.)

Tutorial: A dongle is essentially a physical key used for copy protection of software; that is, the program will not run unless the matching dongle is attached. When the software runs, it periodically queries the dongle and quits if the dongle does not reply with the proper authentication information. Dongles were originally constructed as an EPROM (erasable programmable read- only memory) to be connected to a serial input-output port of a personal computer.