lexicology vs toponymy

lexicology and toponymy both are nouns.

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

As nouns, toponymy is a hyponym of lexicology; that is, toponymy is a word with a more specific, narrower meaning than lexicology:

lexicology: the branch of linguistics that studies the lexical component of language
toponymy: the branch of lexicology that studies the place names of a region or a language

Other hyponyms of lexicology include onomastics, toponomy.

lexicology (noun)	toponymy (noun)
the branch of linguistics that studies the lexical component of language	the branch of lexicology that studies the place names of a region or a language
	the nomenclature of regional anatomy