In jurisprudence, surplusage is a useless statement completely irrelevant to the cause. Surplusages may be included in any declaration, plea, or claim. According to LectLaw [1]
if a man in his declaration, plea, etc., make mention of a thing which need, not be stated, but the matter set forth is grammatically right, and perfectly sensible, no advantage can be taken on demurrer. When, by an unnecessary allegation the plaintiff shows he has no cause of action, the defendant may demur. When the surplusage is not grammatically set right, or it is unintelligible and, no sense at all can be given it, or it be contradictory or repugnant to what is before alleged, the adversary may take advantage of it on special demurrer.
Another use of the term is in statutory interpretation: where one reading of a statute would make one or more parts of the statute redundant and another reading would avoid the redundancy, the other reading is preferred.