diachronic vs historical

diachronic and historical both are adjectives.

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

As adjectives, diachronic and historical are synonyms defined as:

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

diachronic (adjective)	historical (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