1. A security or copy protection device for programs that must be connected to an I/O port of the computer while the program is run. Programs that use a dongle query the port at start-up and at programmed intervals thereafter, and terminate if it does not respond with the expected validation code. Security dongles attempt to combat software theft by ensuring that, while users can still make copies of the program (e.g. for backup), they must buy one dongle for each simultaneous use of the program.

2. Any small hardware device that plugs into a computer port. Examples include USB drives and some Bluetooth transceivers.