The crossword solution for the term REB'S RIVAL is YANK, which is a four-letter word. YANK is a slang term used to refer to a soldier in the United States Army, especially during the American Civil War. The term is derived from the phrase 'Yankee Doodle', which was a popular song during the Revolutionary War. In the context of the American Civil War, YANK refers to a Union soldier, while REB refers to a Confederate soldier. Therefore, YANK is the rival of REB.