historical linguistics vs diachronic linguistics

historical linguistics and diachronic linguistics both are nouns.

Word	Noun	Adjective	Verb	Adverb
historical linguistics	Yes	No	No	No
diachronic linguistics	Yes	No	No	No

As nouns, historical linguistics and diachronic linguistics are synonyms defined as:

historical linguistics and diachronic linguistics: the study of linguistic change

Other synonyms of historical linguistics include diachrony.

historical linguistics (noun)	diachronic linguistics (noun)
the study of linguistic change	the study of linguistic change