Very small measurement used by toolmakers & engineers, or roughly .0001 or one ten thousandth of an inch. Actual size may vary between shops, but always much less than a pussy hair
. About one tenth of a skosh
which is equal to approximately one nudge of the dial.
Note: When specified, generally regarded as smaller than the smallest one can measure or produce, since while most people intuitively understand that a gnat's ass
is pretty small, few have actually bothered a gnat to measure one.
"Clearance is clearance, Clarance, but that die is only missing by a gnat's ass".