Real Dictionary

doubt
If you have doubt or doubts about something, you feel uncertain about it and do not know whether it is true or possible. If you say you have no doubt about it, you mean that you are certain it is true.
verb: If you doubt whether something is true or possible, you believe that it is probably not true or possible.

doubt in American English
a wavering of opinion or belief; lack of conviction; uncertainty
to be uncertain about; question; feel distrust of
to be uncertain in opinion or belief; be undecided
intransitive verb: to be uncertain about something; be undecided in opinion or belief
noun: a feeling of uncertainty about the truth, reality, or nature of something
transitive verb: to be uncertain about; consider questionable or unlikely; hesitate to believe