The party against whom an appeal is filed.  The appellee usually seeks affirmance of the lower court's decision.  By contrast, the appellant is the party who filed the appeal.

Suppose P sues D, and wins.  D files an appeal.  P is the appellee, and D is the appellant.  If D wins the appeal, and P appeals, the roles are reversed.  D becomes the appellee, and P is the appellant.

See Appellate Procedure.