The Collaborative International Dictionary of English v.0.48:

Terra incognita \Ter"ra in*cog"ni*ta\ [L.]
   An unknown land; unexplored country.

         The enormous tracts lying outside China proper, still
         almost terrae incognitae.                --A. R.
                                                  Colquhoun.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

terra incognita
    n 1: an unknown and unexplored region; "they came like angels
         out the unknown" [syn: unknown, unknown region, terra
         incognita]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "terra incognita":
   dark horse, enigma, frontier, frontiers of knowledge,
   matter of ignorance, mystery, n, puzzle, riddle, sealed book,
   the incalculable, the strange, the unfamiliar, the unknowable,
   the unknown, unexplored ground, unexplored territory,
   unknown quantity, x, z