indubitable UK /ɪnˈdjuːbɪtəbəl/US /ɪnˈdubɪtəbəl/adjective 1noun 1forms 3 adjective1 •Clearly true; providing no possibility of doubt.The indubitable effect of the potion convinced many nonbelievers. noun1 •That which is indubitable.If we build logically upwards from a few indubitables, the whole system must remain correct. formsindubitables · more indubitable · most indubitable