Synchrony and diachrony are two different and complementary viewpoints in linguistic analysis. A synchronic approach (from Greek συν- "together" and χρόνος "time") considers a language at a moment in time without taking its history into account. Synchronic linguistics aims at describing a language at a specific point of time, usually the present. By contrast, a diachronic approach (from δια- "through" and χρόνος "time") considers the development and evolution of a language through history. Historical linguistics is typically a diachronic study.
The concepts were theorized by the Swiss linguist Ferdinand de Saussure, professor of general linguistics in Geneva from 1896 to 1911, and appeared in writing in his posthumous Course in General Linguistics published in 1916. In contrast with most of his predecessors, who focused on historical evolution of languages, Saussure emphasized the primacy of synchronic analysis of languages to understand their inner functioning, though never forgetting the importance of complementary diachrony. This dualistic opposition has been carried over into philosophy and sociology, for instance by Roland Barthes and Jean-Paul Sartre. Jacques Lacan also used it for psychoanalysis. Prior to de Saussure, many similar concepts were also developed independently by Polish linguists Jan Baudouin de Courtenay and Mikołaj Kruszewski of the Kazan school, who used the terms statics and dynamics of language.
In 1970 Eugenio Coșeriu, revisiting De Saussure's synchrony and diachrony distinction in the description of language, coined the terms diatopic, diastratic and diaphasic to describe linguistic variation.