In England & Wales and most of the Commonwealth, the title of an appellate court is the Court of Appeal. The word 'appeal' is singular because it describes the purpose of the court in contrast to the Court of Justice (i.e. justice has been done by the court below, and the action now is that of appeal, similarly to the nomenclature of a Court of Cassation, a Court for the Correction of Errors, etc).
In the United States, some states follow the 'traditional' nomenclature—eg, the largest, California, has a Court of Appeal.
However, the federal courts, and many states, instead add an additional 's' to the title of appellate courts. Why is this, and where did it originate from?