Definition of doubt :
1. A fluctuation of mind arising from defect of knowledge or evidence; uncertainty of judgment or mind; unsettled state of opinion concerning the reality of an event, or the truth of an assertion, etc.; hesitation.
2. Difficulty expressed or urged for solution; point unsettled; objection.
3. Suspicion; fear; apprehension; dread.
4. To fill with fear; to affright.
5. To question or hold questionable; to withhold assent to; to hesitate to believe, or to be inclined not to believe; to withhold confidence from; to distrust; as, I have heard the story, but I doubt the truth of it.
6. To suspect; to fear; to be apprehensive of.
7. To suspect; to fear; to be apprehensive.
8. To waver in opinion or judgment; to be in uncertainty as to belief respecting anything; to hesitate in belief; to be undecided as to the truth of the negative or the affirmative proposition; to b e undetermined.
9. Uncertainty of condition.
beyond the shadow of a doubt, misdoubt, mistrust, conviction, motion, hesitancy, ambiguity, bashfulness, irresolution, belief, timidity, precariousness, insecurity, vacillation, wavering, uncertainness, scruple, interrogation, indecision, inquiry, certain, certainly, suspense, reluctance, diffidence, unquestionably, bewilderment, demur, difficulty, interrogative sentence, enquiry, quandary, surmise, surely, query, head, dilemma, doubtfulness, perplexity, reservation, dubiety, ambivalence, dubiousness, interrogative
disbelief (part of speech: noun)
indiscrimination (part of speech: noun)
questioning (part of speech: verb)
doubt (part of speech: verb)