The Collaborative International Dictionary of English v.0.48:

Baptist \Bap"tist\ (b[a^]p"t[i^]st), n. [L. baptista, Gr.
   baptisth`s.]
   [1913 Webster]
   1. One who administers baptism; -- specifically applied to
      John, the forerunner of Christ. --Milton.
      [1913 Webster]

   2. One of a denomination of Christians who deny the validity
      of infant baptism and of sprinkling, and maintain that
      baptism should be administered to believers alone, and
      should be by immersion. See Anabaptist.
      [1913 Webster]

   Note: In doctrine the Baptists of this country [the United
         States] are Calvinistic, but with much freedom and
         moderation. --Amer. Cyc.
         [1913 Webster]

   Freewill Baptists, a sect of Baptists who are Arminian in
      doctrine, and practice open communion.

   Seventh-day Baptists, a sect of Baptists who keep the
      seventh day of the week, or Saturday, as the Sabbath. See
      Sabbatarian. The Dunkers and Campbellites are also
      Baptists.
      [1913 Webster] Baptistery

WordNet (r) 3.0 (2006):

Baptist
    n 1: follower of Baptistic doctrines