The Collaborative International Dictionary of English v.0.48:

Preacher \Preach"er\ (pr[=e]ch"[~e]r), n. [Cf. OF. preeschierre,
   prescheur, F. pr[^e]cheur, L. praedicator.]
   1. One who preaches; one who discourses publicly on religious
      subjects.
      [1913 Webster]

            How shall they hear without a preacher? --Rom. x.
                                                  14.
      [1913 Webster]

   2. One who inculcates anything with earnestness.
      [1913 Webster]

            No preacher is listened to but Time.  --Swift.
      [1913 Webster]

   Preacher bird (Zool.), a toucan.
      [1913 Webster]

WordNet (r) 3.0 (2006):

preacher
    n 1: someone whose occupation is preaching the gospel [syn:
         preacher, preacher man, sermonizer, sermoniser]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "preacher":
   Boanerges, chalk talker, churchman, circuit rider, clergyman,
   clergywoman, cleric, clerical, clerk, discourser, divine,
   ecclesiastic, evangelist, expositor, expounder, homilist, lector,
   lecturer, minister, parson, praelector, preaching friar, predicant,
   predikant, pulpitarian, pulpiteer, pulpiter, reader, reverend,
   sermoner, sermonist, sermonizer, tub-thumper