The Collaborative International Dictionary of English v.0.48:

Appellant \Ap*pel"lant\, a. [L. appellans, p. pr. of appellare;
   cf. F. appelant. See Appeal.]
   Relating to an appeal; appellate. "An appellant
   jurisdiction." --Hallam.
   [1913 Webster]

   Party appellant (Law), the party who appeals; appellant; --
      opposed to respondent, or appellee. --Tomlins.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Appellant \Ap*pel"lant\, n.
   1. (Law)
      (a) One who accuses another of felony or treason. [Obs.]
      (b) One who appeals, or asks for a rehearing or review of
          a cause by a higher tribunal.
          [1913 Webster]

   2. A challenger. [Obs.] --Milton.
      [1913 Webster]

   3. (Eccl. Hist.) One who appealed to a general council
      against the bull Unigenitus.
      [1913 Webster]

   4. One who appeals or entreats.
      [1913 Webster]

WordNet (r) 3.0 (2006):

appellant
    adj 1: of or relating to or taking account of appeals (usually
           legal appeals); "appellate court" [syn: appellate,
           appellant]
    n 1: the party who appeals a decision of a lower court [syn:
         appellant, plaintiff in error]

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

APPELLANT, practice. He who makes an appeal from one jurisdiction to
another.