means A detailed, explicit description of how something should be made, behave, or perform — the exact requirements something must meet to count as correct.

from From the Latin 'specificare,' to make of a particular kind, built from 'species' (kind, sort, appearance — literally 'a look,' from 'specere,' to see) plus '-ficare,' to make. So at root, to specify is to spell out exactly what kind of thing you mean — and 'specification' is the act of pinning down that look in words. English took it up by the 17th century, and the clipped 'spec' is a modern engineer's shorthand.