Improbable
Part of speech: adjective
Definition: Not likely to be true. Not likely to happen.
Example sentence: Remember, I'm someone who got fired from Disney and eight days later started the first studio in 65 years with two of the most brilliant, successful people in the history of the entertainment business, doing something everyone said was somewhere between improbable and impossible. I'm afraid that's exactly what I love doing.
Unbelievable
Part of speech: adjective
Definition: Incredible; not to be believed.Implausible or improbable.
Example sentence: Let's drink to the spirit of gallantry and courage that made a strange Heaven out of unbelievable Hell, and let's drink to the hope that one day this country of ours, which we love so much, will find dignity and greatness and peace again.