We've heard the "USS," as in USS Enterprise
, explained on screen on more than one occasion as "united space ship" or "united starship." Has any character in actual on-screen dialogue, or in an on-screen written reference such as computer display, explained what "NCC" stands for in Starfleet context? Offhand, I can't think of any such reference.
Of course, we know that in real life, "NC" was used in the prefixes of airplane numbers that Matt Jeffries and Gene Roddenberry were familiar with from the military, and that Jeffries simply added an additional "C." But has there ever been an in-universe explanation?