Sudden means happening quickly and unexpectedly, without warning or preparation time. When something is sudden, it catches you off guard: a sudden clap of thunder during a quiet afternoon, a sudden change in your teacher's mood, or a sudden realization that you left your homework at home.

The word carries a sense of surprise and speed combined. A sudden rainstorm doesn't gradually build up with dark clouds you can watch for an hour. It just arrives, drenching you before you can find shelter. A sudden stop while riding your bike throws you forward because your body didn't have time to prepare.

Not everything fast is sudden. A sprinter bursting from the starting blocks runs fast, but it's not sudden because everyone expected it. But if that same runner suddenly veered off the track, that would be sudden because nobody saw it coming.

When writers say something happened “all of a sudden,” they're emphasizing how unexpected and quick it was. You might be reading peacefully when all of a sudden your little brother bursts into your room. The phrase sudden death in sports means the game ends immediately when someone scores, leaving no time for a comeback.