This is the name given to any system, mechanical or electronic, that generates a waveform, acoustic or electric, that repeats at a fixed or variable frequency. The frequency of the oscillation defines the frequency of the generated waveform.
This is a recording process used in multitrack recording. Basically, the performer will listen to previously recorded tracks while they record another part to a different track. The tracks are then combined, usually to a stereo pair, during the mixdown process.