Your book is building up to something intense, right? Your two favorite characters are about to kiss. Or the hero is only pages away from slaying the dragon. Or the detective is just steps behind the diamond thief. Here it comes, the big payoff you've been waiting for!
But then, all of a sudden, there's a shift. And everything changes. Whatever event you were anticipating—the smooch, the dead dragon, the thrilling capture—doesn't happen.
Yeah, that's anticlimax. It's just a fancy term for a disappointing end to all the hullabaloo. It's the bummer, the let-down, the womp womp.