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).
Phrases and idioms:
for a certainty beyond the possibility of doubt.
Etymology: ME f. AF certainteacute, OF -eteacute (as CERTAIN)
Useful english dictionary. 2012.