An unknown or unexplored region. Often figurative.

An indeterminable point or problem.

XYZ: used to denote some thing or person unknown or undetermined (cf. 3).

gen. An unknown thing; esp. an unknown factor, variable, or situation.

With plural agreement. Unknown things or places.

Those aspects of a situation which are unknown at present; (Military) the enemy position or activities.

Used, with qualifying adjectives, to form the names of medicinal and other earths, boles, and the like… = terra incognita, n.

A situation, condition, or state of affairs very different in nature or character from one's usual experience; something which is unfamiliar…