1, 2. Doubtless, undoubtedly, indubitably, unquestionably are adverbs that express certainty. Doubtless, although it sometimes denotes an absolute degree of certainty, more often means “probably,” “presumably,” or “no doubt”: She will doubtless accept the offer. Undoubtedly means “beyond doubt”: undoubtedly the most prolific and popular composer of his time. Indubitably and unquestionably both affirm an unassailable conviction; they differ only in tone, indubitably being appropriate to more formal or learned discourse and unquestionably appropriate in a wider range of contexts and styles: an indubitably (or unquestionably) accurate transliteration of the hieroglyphic text; unquestionably the hottest running back in the league.