Appellant

The party who appeals a district court's decision, usually seeking reversal of that decision.

Additional Sources

Answers.com

One who appeals a court decision.

Law.com Dictionary

The party who appeals a trial court decision he/she/it has lost.

Lawyers.com

A person or party who appeals a court's judgment.

The Free (Legal) Dictionary

A person who, dissatisfied with the judgment rendered in a lawsuit decided in a lower court or the findings from a proceeding before an Administrative Agency, asks a superior court to review the decision.

An appellant, sometimes called the petitioner, must demonstrate sufficient grounds for appeal, which are usually specified by statute, in order to challenge the judgment or findings.

Wikipedia

The appellant is the party who, having lost part or all their claim in a lower court decision, is appealing to a higher court to have their case reconsidered. This is usually done on the basis that the lower court judge erred in the application of law, but it may also be possible to appeal on the basis of court misconduct, or that a finding of fact was entirely unreasonable to make on the evidence.