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]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "niche":
   alcove, asylum, bay, bolt-hole, cache, carrel, concealment, corner,
   cove, cover, covert, coverture, cranny, cubby, cubbyhole, cubicle,
   dark corner, den, dugout, foxhole, funk hole, hideaway, hideout,
   hidey hole, hiding, hiding place, hole, hollow, inglenook, lair,
   nook, oriel, pitchhole, place, position, recess, recession, refuge,
   retreat, roomlet, sanctuary, secret place, slot, snuggery, stash,
   undercovert