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]