A letdown is the flat feeling — or the event itself — when something is not as good as you hoped. The film, the concert, the holiday, the ending of a book: you built it up in your head, and the real thing was smaller. People say What a letdown after a match their team should have won, or after a talk that sounded exciting on the poster. It comes from let down, the verb that means to disappoint someone (Don’t let me down). The noun is often one word, letdown; you will also see let-down. It is everyday English, a little informal, and it always needs that gap between hope and what arrived. A thing can be fine on its own and still be a letdown if the promise was bigger.