An irrevocable decision, step, or change cannot be taken back, changed, or stopped once it has happened. If you sign an irrevocable agreement, you cannot later say you have changed your mind. The word is formal and a little legal; you will see it in contracts, in news about courts, and in serious talk about damage that cannot be undone. Though actually, everyday English often just says you cannot take it back, or it is final. Irrevocable is the heavy stamp on that idea. It does not mean irresistible (hard not to want), and it does not mean you cannot speak.