To justify something means to give good reasons for why it's right, necessary, or acceptable. When you justify staying up past your bedtime, you might explain that you need extra time to finish an important school project. When a teacher justifies a difficult homework assignment, she explains how it will help you learn something important.

Justifying requires actual reasons that make sense and show understanding. If you broke your sister's toy, saying “I felt like it” doesn't justify your action, but explaining “I accidentally sat on it because I didn't see it on the chair” does. The difference is that a real justification shows why something happened for understandable reasons by explaining the circumstances or necessity behind an action.

In writing and formatting, justify also has a technical meaning: to align text so both the left and right edges are straight and even, like in many books and newspapers. You might justify the margins of a document to make it look more professional.

People sometimes use justify when they mean rationalize or make excuses for questionable behavior. You'll hear someone say “there's no way to justify cheating” or “nothing can justify being cruel.” This means no good reason exists for those actions, no matter what explanation someone might offer.