diachronic
adjective
dia·chron·ic ˌdī-ə-ˈkrä-nik 
: of, relating to, or dealing with phenomena (as of language or culture) as they occur or change over a period of time
diachronically
ˌdī-ə-ˈkrä-ni-k(ə-)lē
adverb 
Word History
Etymology
borrowed from French diachronique, from diachronie diachrony + -ique -ic entry 1
First Known Use
1922, in the meaning defined above