Any device that moves a medium past a head or stylus at a supposedly constant speed drifts slightly. Slow speed variations, below about four cycles per second, are heard as wow, a seasick bending of pitch; faster ones are flutter, a subtle shimmer or roughness. The two are usually measured together as a percentage: around 0.03% for professional tape machines, about 0.08% for the best cassette decks and 0.2% or more for average cassette and car players. Turntables suffer mainly from wow.
Lo-fi producers often add wow and flutter on purpose, with plug-ins or real cassette decks, to make keys and chords sway as if played from a worn tape. A little goes a long way: too much quickly sounds out of tune.