To disbelieve means to think something isn't true or refuse to accept it as real. When you disbelieve a story, you don't just have doubts: you actively think it's false. If your friend claims they saw a dinosaur in their backyard, you would probably disbelieve them because you know non-bird dinosaurs went extinct millions of years ago.

Disbelief is stronger than simple doubt or uncertainty. Doubt means you're unsure and want more information. Disbelief means you've decided something is wrong or impossible. A teacher might disbelieve a student's excuse if the explanation seems obviously made up. A detective might disbelieve a suspect's alibi if the evidence contradicts their story.

Sometimes people use “disbelieve” to mean they can't quite accept something surprising but true: “I can't believe the summer is already over!” But the core meaning involves rejecting something as untrue. Scientists disbelieved the theory that the Earth was flat once they had proof it was round. Throughout history, people have had to overcome their disbelief when facing startling new discoveries, from germs causing disease to humans landing on the moon.