UNC is short for UNCharted world. As most of Andromeda is uncharted, most explorable areas you visit in the game are occasionally referred to as UNCs. For comprehensive maps of these explorable areas, see the Exploration chapter.
Major UNCs
Gameplay-wise, ”major“ UNCs are vast open-world areas with myriad locations, activities, and missions. While there are currently four major UNCs, expect to spend numerous hours or days exploring each one.
Minor UNCs
Minor UNCs are much more compact, generally compromising of a handful of missions (although each has an extensive mission chain, giving you a tour of most of the area).