The Collaborative International Dictionary of English v.0.48:

Celebrant \Cel"e*brant\, n. [L. celebrans, p. pr. of celebrare.
   See Celebrate.]
   One who performs a public religious rite; -- applied
   particularly to an officiating priest in the Roman Catholic
   Church, as distinguished from his assistants.
   [1913 Webster]

WordNet (r) 3.0 (2006):

celebrant
    n 1: a person who is celebrating [syn: celebrant,
         celebrator, celebrater]
    2: an officiating priest celebrating the Eucharist