The party who appeals a lower court's decision in a higher court. The appellant seeks reversal or modification of the decision. By contrast, the appellee is the party against whom the appeal is filed.
Suppose P sues D, and wins. D files an appeal. D is the appellant, and P is the appellee. If D wins the appeal, and P appeals, the roles are reversed. P becomes the appellant and D is the appellee.
See Appellate Procedure.