The Collaborative International Dictionary of English v.0.48:

Plaintiff \Plain"tiff\, n. [F. plaintif making complaint,
   plaintive; in Old French equiv. to plaignant complainant,
   prosecutor, fr. plaindre. See Plaint, and cf. Plaintive.]
   (Law)
   One who commences a personal action or suit to obtain a
   remedy for an injury to his rights; -- opposed to
   defendant.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Plaintiff \Plain"tiff\, a.
   See Plaintive. [Obs.] --Prior.
   [1913 Webster]

WordNet (r) 3.0 (2006):

plaintiff
    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:

47 Moby Thesaurus words for "plaintiff":
   accessory, accusant, accuser, allegator, appellant, bitter-ender,
   brawler, claimant, complainant, defendant, delator, diehard,
   disputant, dissentient, dissident, impeacher, impugner, indictor,
   informer, intransigent, irreconcilable, last-ditcher, libelant,
   litigant, litigationist, litigator, naysayer, negativist,
   noncooperator, objector, obstructionist, obstructive, opposer,
   oppositionist, panel, parties litigant, party, petitioner,
   prosecutor, protester, quarreler, resister, scrapper, suitor,
   the prosecution, witness, wrangler



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

PLAINTIFF, practice. He who, in a personal action, seeks a remedy for an
injury to his rights. Ham. on Parties, h.t.; 1 Chit. Pl. Index, h.t.; Chit.
Pr. Index, h.t.; 1 Com. Dig. 36, 205, 308.
     2. Plaintiffs are legal or equitable. The legal plaintiff is he in whom
the legal title or cause of action is vested. The equitable plaintiff is he
who, not having the legal title, yet, is in equity entitled to the thing
sued for; for example, when a suit is brought by Benjamin Franklin for the
use of Robert Morris, Benjamin Franklin is the legal, and Robert Morris the
equitable plaintiff. This is the usual manner of bringing suit, when the
cause of action is not assignable at law, but is so in equity. Vide Bouv.
Inst. Index, h.t.; Parties to Actions.