one-ninth vs ninth

one-ninth and ninth both are nouns.

Word	Noun	Adjective	Verb	Adverb
one-ninth	Yes	No	No	No
ninth	Yes	Yes	No	No

As nouns, one-ninth and ninth are synonyms defined as:

one-ninth (noun)	ninth (noun)
one part in nine equal parts	one part in nine equal parts
	position nine in a countable series of things

one-ninth (adjective)	ninth (adjective)
	coming next after the eighth and just before the tenth in position