The Collaborative International Dictionary of English v.0.48:

Niche \Niche\ (n[i^]ch), n. [F., fr. It. nicchia, prop., a
   shell-like recess in a wall, fr. nicchio a shellfish, mussel,
   fr. L. mytilus.]
   A cavity, hollow, or recess, generally within the thickness
   of a wall, for a statue, bust, or other erect ornament.
   Hence, any similar position, literal or figurative.
   [1913 Webster]

         Images defended from the injuries of the weather by
         niches of stone wherein they are placed. --Evelun.
   [1913 Webster]

WordNet (r) 3.0 (2006):

niche
    n 1: a position particularly well suited to the person who
         occupies it; "he found his niche in the academic world"
    2: a small concavity [syn: recess, recession, niche,
       corner]
    3: an enclosure that is set back or indented [syn: recess,
       niche]
    4: (ecology) the status of an organism within its environment
       and community (affecting its survival as a species) [syn:
       niche, ecological niche]