Line level is the specified strength of an audio signal used to transmit analog audio between components such as CD and DVD players, television sets, audio amplifiers, and mixing consoles.

Line level sits between other levels of audio signals. There are weaker signals such as those from microphones (microphone level or mic level) and instrument pickups (instrument level), and stronger signals, such as those used to drive headphones and loudspeakers (speaker level). The "strength" of these various signals does not necessarily refer to the output voltage of the source device; it also depends on its output impedance and output power capability.

Consumer electronic devices concerned with audio (for example sound cards) often have a connector labeled line in and/or line out. Line out provides an audio signal output and line in receives a signal input.

The line in/out connections on consumer-oriented audio equipment are typically unbalanced, and connected with a stereo RCA jacks, or with a 3.5 mm (0.14 inch, but commonly called "eighth inch") 3-conductor TRS minijack connector providing ground, left channel, and right channel.

Professional equipment commonly uses balanced connections on 6.35 mm (1/4 inch) TRS phone jacks or XLR connectors. Professional equipment may also use unbalanced connections with (1/4 inch) TS phone jacks.