What is the best definition of the word tribunal ?

A tribunal, generally, is any person or institution with authority to judge, adjudicate on, or determine claims or disputes—whether or not it is called a tribunal in its title. For example, an advocate who appears before a court with a single judge could describe that judge as 'their tribunal'.
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Tribunal is any person or institution with authority to judge, adjudicate on, or determine claims or disputes whether or not it is called a tribunal in its title.