The Collaborative International Dictionary of English v.0.48:

Baptistery \Bap"tis*ter*y\,Baptistry \Bap"tis*try\, n.; pl.
   Baptisteries, Baptistries. [L. baptisterium, Gr.
   baptisth`rion: cf. F. baptist[`e]re.] (Arch.)
      (a) In early times, a separate building, usually
          polygonal, used for baptismal services. Small churches
          were often changed into baptisteries when larger
          churches were built near.
      (b) A part of a church containing a font and used for
          baptismal services.
          [1913 Webster]

WordNet (r) 3.0 (2006):

baptistery
    n 1: bowl for baptismal water [syn: baptismal font,
         baptistry, baptistery, font]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "baptistery":
   Easter sepulcher, affusion, ambry, apse, aspergation, aspersion,
   baptism, baptismal gown, baptismal regeneration, baptizement,
   blindstory, chancel, choir, chrismal, christening, cloisters,
   confessional, confessionary, crypt, diaconicon, diaconicum, font,
   immersion, infusion, nave, porch, presbytery, rood loft,
   rood stair, rood tower, sacrarium, sacristy, sprinkling,
   total immersion, transept, triforium, vestry