Sockets found on very few camcorders that allow you to record pictures and sound directly from another video player. In audio terms this can be used to refer to the sound level of the input - line input is appropriate for a signal from a tape player, mixer or other equipment as opposed to mic input which is designed for the much lower level of signal from a microphone. Note that a radio mic receiver may have a "line" level output.
An input device that accepts a line level signal
an input designed to receive line-outputs, at either standard, and probably any signal that exists between the two standards
a socket on most VCRs and some camcorders which accepts vision or sound signals from an auxiliary source such as another VCR or audio cassette recorder.
Any input designed to accept a line-level input signal.