Feeling uncertain about something; not sure. She was doubtful about the success of the new project.
Not likely to be true or to happen; questionable. The story he told sounded doubtful to everyone who heard it.
Causing or giving rise to doubt; suspicious. The evidence presented in court was doubtful and did not convince the jury.
Not yet decided or settled; uncertain in outcome. With the votes still being counted, the result of the election is doubtful.