It holds no exact translation into either the US Standard or the Metric Systems, and is an extremely variable term. The closest possible definition is: an approximation of an extremely small increment.
"Holy Shit! I just caught one in the plate! Had that been a Cunt Hair over to the left, I'd be a dead man, Sir!"
Carpenter Says "I need to bang that stud a cunt hair to the left" .25 inch
Astronmer says That Astroid missed the planet by a cunt hair" 60,000 miles