The adjective improbable also means statistically unlikely to happen. You might be afraid to fly, but the odds of a plane crash are so low that such an event is improbable. Improbable also means something that is so outrageous that you'd never admit you believed in it.

What type of word is improbable?

Not likely to be true. "It's highly improbable that aliens abducted you."

Does improbable mean doubtful?

questionable, doubtful, implausible.