A loopback may refer to any of the following:
1. With a hardware device, loopback refers to a physical connection between two devices of the same type or loops back to itself. Loopback devices test a connection for a link, data transmission capabilities, or verify if the connection works.
2. With a test, a loopback test is one or more signals sent from a device that is looped back (returned) to the same device. By running and passing a loopback test, a person can identify if the device can send and receive data. If a loopback test fails, it might indicate failing hardware. Examples of where a loopback test may be used are with network cards, network routers, and serial ports.
3. With a network address, see localhost.