The Collaborative International Dictionary of English v.0.48:

Columbarium \Col`um*ba"ri*um\, n.; pl. L. Columbaria [L. See
   Columbary.] (Rom. Antiq.)
   (a) A dovecote or pigeon house.
   (b) A sepulchral chamber with niches for holding cinerary
       urns.
       [1913 Webster]

WordNet (r) 3.0 (2006):

columbarium
    n 1: a birdhouse for pigeons [syn: dovecote, columbarium,
         columbary]
    2: a niche for a funeral urn containing the ashes of the
       cremated dead [syn: columbarium, cinerarium]
    3: a sepulchral vault or other structure having recesses in the
       walls to receive cinerary urns