A more thorough explanation:

A cross-appeal is a legal request made by the party who did not file the initial appeal, asking a higher court to review a decision made by a lower court. The difference between an appeal and a cross-appeal is based on who filed the request for review first. The party who files the request first is called the petitioner or appellant, and their request is an appeal. If the opposing party also wants to request a review of the lower court's decision, their request is called a cross-appeal.

In a civil trial, the plaintiff files a claim against the defendant. If the defendant has claims against the plaintiff, it may file a counterclaim. Similarly, in an appeal, the party who files the request first is called the appellant, and if the opposing party also wants to request a review of the lower court's decision, their request is called a cross-appeal.

For example, in the case of Ulin v. Gallery 528 Fed.Appx. 748 (2013), the appellant filed an appeal against the lower court's decision. The appellee then filed a cross-appeal, requesting the higher court to review some aspect of the lower court's decision. This is an example of a cross-appeal.

Note that a cross-appeal only occurs when the appellee requests that the higher court review some aspect of the lower court's decision, and not just to uphold the lower court's decision.