What is a legal error by a lower court that does not require reversal is called a(n)?

A legal error by a lower court that does not require reversal is called a "harmless error." This means that the error did not affect the outcome of the case significantly and, therefore, does not warrant a new trial or a change in the court's decision. Courts typically assess whether the error had a substantial impact on the rights of the parties involved before determining if it is harmless.