What are loop-back plugs?

Exact definition of loop-back plugs will depend on what application we are talking about. Mostly these are connector plugs with a metal inside to short two electrical signals. There can be loopback plugs for different applications. These can be for a serial port or an Ethernet port or loopback plugs for specific signals on a connector or on a printed circuit board. These are very useful for debugging and testing an electronics problem. E.g. one can do a loopback on PC serial port's Transmit and Receive (Tx and Rx) lines using a loopback plug or a metal short to see if the serial port is working fine. Similarly Ethernet ports can be tested with a loop-back plug made for the purpose. - Neeraj Sharma