adjective. Of or concerned with phenomena, such as linguistic features, as they change through time.

adjective. Occurring or changing along with time.

adjective. used of the study of a phenomenon (especially language) as it changes through time

"Strawson goes on to identify two personality types, which he calls the diachronic type, the kind of person disposed to conceive of themselves connected to both their past and future selves, and the episodic type, which is the kind of person who does not tend to conceive of their momentary self as part of a chain of selves stretching into the past and future."

synchronic