The Collaborative International Dictionary of English v.0.48:

Complainant \Com*plain"ant\ (k[o^]m*pl[=a]n"ant), n. [F.
   complaignant, p. pr. of complaindre.]
   1. One who makes complaint.
      [1913 Webster]

            Eager complainants of the dispute.    --Collier.
      [1913 Webster]

   2. (Law)
      (a) One who commences a legal process by a complaint.
      (b) The party suing in equity, answering to the plaintiff
          at common law.
          [1913 Webster]

                He shall forfeit one moiety to the use of the
                town, and the other moiety to the use of the
                complainant.                      --Statutes of
                                                  Mass.
          [1913 Webster]

WordNet (r) 3.0 (2006):

complainant
    n 1: a person who brings an action in a court of law [syn:
         plaintiff, complainant] [ant: defendant, suspect]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "complainant":
   accusant, accuser, allegator, appellant, bellyacher, claimant,
   complainer, crab, crank, croaker, delator, faultfinder, frondeur,
   griper, grouch, grouser, growler, grumbler, impeacher, impugner,
   indictor, informer, kicker, kvetch, libelant, malcontent, murmurer,
   mutterer, party, petitioner, plaintiff, prosecutor,
   querulous person, reactionary, reactionist, rebel, sorehead,
   suitor, the prosecution, whiner



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

COMPLAINANT. One who makes a complaint. A plaintiff in a suit in chancery is
so called.