toponymy vs nomenclature

toponymy and nomenclature both are nouns.

Word	Noun	Adjective	Verb	Adverb
toponymy	Yes	No	No	No
nomenclature	Yes	No	No	No

As nouns, nomenclature is a hypernym of toponymy; that is, nomenclature is a word with a broader meaning than toponymy:

toponymy: the nomenclature of regional anatomy
nomenclature: a system of words used to name things in a particular discipline

Other hypernyms of toponymy include language, terminology.

toponymy (noun)	nomenclature (noun)
the branch of lexicology that studies the place names of a region or a language	a system of words used to name things in a particular discipline
the nomenclature of regional anatomy