[determiner], [pronoun], [adjective], [adverb] [not gradable], nounthe fifteenth lapHer birthday is on the fifteenth of May.You're fifteenth on the list.He's ranked fifteenth in the world.A fifteenth is one of fifteen equal parts of something.
FIFTEENTH
Meaning of FIFTEENTH in English
Cambridge English vocab. Кембриджский английский словарь. 2012