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

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "chaplain":
   DD, Doctor of Divinity, Grand Penitentiary, Holy Father, Holy Joe,
   abbe, abuna, antipope, archbishop, archdeacon, archpriest, bishop,
   bishop coadjutor, canon, cardinal, cardinal bishop,
   cardinal deacon, cardinal priest, churchman, clergyman, cleric,
   clerical, clerk, coadjutor, curate, cure, dean, diocesan, divine,
   ecclesiarch, ecclesiastic, exarch, hierarch, high priest,
   man of God, metropolitan, military chaplain, minister, padre, papa,
   parson, pastor, patriarch, penitentiary, pontiff, pope, prebendary,
   prelate, primate, rector, reverend, rural dean, servant of God,
   shepherd, sky pilot, subdean, suffragan, supply clergy,
   supply minister, the Reverend, the very Reverend, tonsured cleric,
   vicar



Bouvier's Law Dictionary, Revised 6th Ed (1856):

CHAPLAIN. A clergyman appointed to say prayers and perform divine service.
Each house of congress usually appoints it own chaplain.