Reason mean a cause, explanation, or justification for an action or event or the power of the mind to think, understand, and form judgments by a process of logic. A reason explains why you do something. Reason usually has to do with thought and logic, as opposed to emotion. If people think you show good reason, or are reasonable, it means you think things through. If people think you have a good reason for doing something, it means you have a motive that makes sense.