-synchronic: Linguistics. [tr. F. synchronique (F. de Saussure a 1913, in Cours de linguistique générale (1916) iii. 117).] Pertaining to or designating a method of linguistic study concerned with the state of a language at one time, past or present; descriptive, as opposed to historical or diachronic. Also transf. in Anthropology, etc.

-diachronic: Linguistics. [tr. F. diachronique (F. de Saussure a 1913, in Cours de linguistique générale (1916) iii. 120).] Pertaining to or designating a method of linguistic study concerned with the historical development of a language; historical, as opposed to descriptive or synchronic. Also transf., in Anthropology, etc. Hence dia{sm}chronically adv.; di{sm}achrony.

* This information has been taken from OED