The Collaborative International Dictionary of English v.0.48:

Chaplain \Chap"lain\, n. [F. chapelain, fr. LL. capellanus, fr.
   capella. See Chapel.]
   1. An ecclesiastic who has a chapel, or who performs
      religious service in a chapel.
      [1913 Webster]

   2. A clergyman who is officially attached to the army or
      navy, to some public institution, or to a family or court,
      for the purpose of performing divine service.
      [1913 Webster]

   3. Any person (clergyman or layman) chosen to conduct
      religious exercises for a society, etc.; as, a chaplain of
      a Masonic or a temperance lodge.
      [1913 Webster]

WordNet (r) 3.0 (2006):

chaplain
    n 1: a clergyman ministering to some institution