diachronic linguistics vs historical linguistics

diachronic linguistics and historical linguistics both are nouns.

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

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

diachronic linguistics and historical linguistics: the study of linguistic change

Other synonyms of diachronic linguistics include diachrony.

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