Loopback (loop-back) refers to the routing of electronic signals, digital data streams, or flows of items back to their originating devices or facilities without intentional processing or modification. This is primarily a means of testing the transmission or transportation infrastructure.
- A communication channel with only one endpoint. Any message transmitted by such a channel is immediately and only received by that same channel.
- Performing transmission tests of access lines from the serving switching center, which usually does not require the assistance of personnel at the served terminal.
- Testing between stations (not necessarily adjacent) where in two lines are used, with the test being done at one station and the two lines interconnected at the distant station. Commonly called loop around when the interconnecting circuit is accessed by dialing.
- A patch cable, applied manually or automatically, remotely or locally, that facilitates a loop-back test.
Virtual Loopback Interface (TCP/IP) 
Implementations of the Internet Protocol Suite include a virtual network interface through which network application clients and servers can communicate when running on the same machine. It is implemented entirely within the operating system's networking software and passes no packets to any network interface controller. Any traffic that a computer program sends to a loopback IP address is simply and immediately passed back up the network software stack as if it had been received from another device.
Unix-like systems usually name this LOopback interface
Network equipment 
Some network equipment uses the term loopback for a virtual interface used for management purposes. Unlike a proper loopback interface, this loopback device is not used to talk with itself.
Such an interface is assigned an address that can be accessed from management equipment over a network but is not assigned to any of the real interfaces on the device. This loopback address is also used for management datagrams, such as alarms, originating from the equipment. The property that makes this virtual interface special is that applications that use it will send or receive traffic using the address assigned to the virtual interface as opposed to the address on the physical interface through which the traffic passes.
Serial interfaces 
A serial communications transceiver can use loopback for testing its functionality. For example, a device's transmit pin connected to its receive pin will result in the device receiving exactly what it transmits. Moving this looping connection to the remote end of a cable adds the cable to this test. Moving it to the far end of a modem link extends the test further. This is a common troubleshooting technique and is often combined with a specialized test device that sends specific patterns and counts any errors that come back (see Bit Error Rate Test). Some devices include built-in loopback capability.
A simple serial interface loopback test, called paperclip test, is sometimes used to identify serial ports of a computer and verify operation. It utilizes a terminal emulator application to send characters, with flow control set to off, to the serial port and receive the same back. For this purpose, a paperclip is used to short pin 2 to pin 3 (the receive and transmit pins) on a standard RS-232 interface using D-subminiature DE-9 or DB-25 connectors.
In telecommunications, loopback (short loop) is a hardware or software method which feeds a received signal or data back to the sender. It is used as an aid in debugging physical connection problems. As a test, many data communication devices can be configured to send specific patterns (such as all ones) on an interface and can detect the reception of this signal on the same port. This is called a loopback test and can be performed within a modem or transceiver by connecting its output to its own input. A circuit between two points in different locations may be tested by applying a test signal on the circuit in one location, and having the network device at the other location send a signal back through the circuit. If this device receives its own signal back, this proves that the circuit is functioning.
A hardware loop is a simple device that physically connects the receive channel to the transmit channel. In the case of a network termination connector such as X.21, this is typically done by simply connecting the pins together in the connector. Media such as optical fiber or coaxial cable, which have separate transmit and receive connectors, can simply be looped together with a single strand of the appropriate medium.
Other applications 
A named pipe can also be seen as a loopback on file system level: While Unix domain sockets and network sockets are using two connected files for Inter-process communication, a named pipe consists of only one file.
The audio systems OSS, ALSA and PulseAudio have also loopback modules for recording the audio output of applications and testing purposes. Unlike physical loopbacks, there is no double analogue/digital conversion and no disruption due to hardware malfunctions.