Doubt is distrust or suspicion. When your kid sister explains that a robber broke in and ate your entire chocolate stash, most likely you feel doubt.
When you're in doubt, you really can't tell what's true and what's false, whether that's a news story or whether you want to marry someone or whether you'll survive Med school. The Latin root of doubt is dubitāre, "to hesitate, waver," and when you doubt something you're wavering: I think I believe you, but maybe I don't. But maybe I do. But maybe not? One thing not to doubt is the silent b. Pronounce: DOWT.