Literally, Latin for "unknown territory".
The blank spaces on the map, places where they used to write "here be dragons" just to fill the gaps. These were the lands where fairy tales took place, where the seventh son of a seventh son had magical powers and the swineherd married the princess.
Our modern maps don't have space for dragons, but we still look for unexplored areas, where our imagination can have free reign. Our terra incognita is more likely to be metaphorical than real now. We find it in new fields of knowledge, new ideas, or new communities. Maps and guidebooks notwithstanding, we even find it when we travel.
Sometimes it's enough that the territory be unknown to us.