fifteenthPronunciation: /fɪfˈtiːnθ/Translate fifteenth | into French | into German | into Italian | into Spanish Definition of fifteenth number 1constituting number fifteen in a sequence; 15th:August the fifteenth the fifteenth century on the fifteenth floor an organ stop sounding a register of pipes two octaves (fifteen notes) above the diapason. 2each of fifteen equal parts into which something is or may be divided. fifteenth in other Oxford dictionaries Definition of fifteenth in the US English dictionary