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]