To thwart means to prevent someone from accomplishing what they're trying to do, or to block a plan from succeeding. When a goalkeeper thwarts a shot on goal, she stops the ball from going in. When rain thwarts your plans for a picnic, it forces you to cancel or change your outdoor plans.

The word carries a sense of active opposition or interference. A security system thwarts burglars by making it too difficult to break in. A clever detective thwarts a criminal's scheme by figuring out the plan before it can succeed. In stories, heroes often thwart villains at the last moment, stopping their dangerous plots just in time.

You can thwart something accidentally (like when unexpected traffic thwarts your plan to arrive early) or deliberately (like when a teacher thwarts students' attempt to skip homework by announcing a surprise quiz). The emphasis is always on prevention: something that seemed possible or likely gets stopped before it can happen. When your careful plans get thwarted, it means something or someone got in the way and prevented them from working out.