n. (pl. -ies) 1 a an undoubted fact. b a certain prospect (his return is a certainty). 2 (often foll. by of, or that + clause) an absolute conviction (has a certainty of his own worth). 3 (often foll. by to + infin.) a thing or person that may be relied on (a certainty to win the Derby). for a certainty beyond the possibility of doubt.
[ ME f. AF certainté, OF -eté (as CERTAIN) ]