What does "UNC" stand for in a mechanical drawing / drafting context?

It’s a screw thread designation: “Unified Coarse”, which refers to the standard for thread sizes and shape used for machine screws in the US. The ”.250” is the nominal diameter of the screw that will fit the hole and the “20” (not ”.20”) is the pitch, or number of threads per inch. In other words, the hole is first drilled with a number 7 drill bit, then tapped using a 1/4–20 tap.