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]