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):

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