To supersede means to take the place of something older, usually because you're better or more up-to-date. When a new edition of a textbook supersedes the old one, the school replaces the worn-out copies with fresh ones that have updated information. When a company's new policy supersedes the old rules, everyone stops following the old way and starts following the new way instead.

The word suggests more than simple replacement. When something supersedes something else, it doesn't just show up: it actively pushes the older thing aside because it's an improvement. Email superseded handwritten letters for most everyday communication because it was faster and more convenient. Smartphones superseded flip phones because they could do so much more. The U.S. Constitution superseded the Articles of Confederation because it created a stronger, more workable government.

You'll often see supersede in formal contexts like business, law, or technology, where keeping track of which rule or version is current really matters. If a teacher announces that tomorrow's quiz supersedes today's homework assignment, they're saying the quiz now takes priority and you should focus your energy there instead. The superseding thing wins out and becomes what people follow or use going forward.