historical vs diachronic

historical and diachronic both are adjectives.

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

As adjectives, historical and diachronic are synonyms defined as:

historical and diachronic: used of the study of a phenomenon (especially language) as it changes through time

historical (adjective)	diachronic (adjective)
used of the study of a phenomenon (especially language) as it changes through time	used of the study of a phenomenon (especially language) as it changes through time
of or relating to the study of history
belonging to the past; of what is important or famous in the past
having once lived or existed or taken place in the real world as distinct from being legendary