Wow and flutter is a word used to describe speed variations within motorised audio devices, this directly translates into pitch vaiations.

Considered undesirabe, as the sound is unpleasant, wow and flutter is measured as a percentage.

An old tape cassette would have a far worse percentage of wow and flutter than say a highly grade turntable.

As moving parts are mechanical they are prone to mechanical wear therefore tolerance would widen with age, thus increasing the chances of wow and flutter.

Small amounts of wow and flutter are not necessarily heard as pitch changes but a "roughness" in the sound.
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Recording Studio Suntans