The study of a language through different periods in history.
Diachronic linguistics is one of the two main temporal dimensions of language study introduced by Swiss linguist Ferdinand de Saussure in his Course in General Linguistics (1916). The other is synchronic linguistics.
Examples and Observations:
- "Diachronic literally means across-time, and it describes any work which maps the shifts and fractures and mutations of languages over the centuries. In gross outline, it is similar to evolutionary biology, which maps the shifts and transformations of rocks. Synchronic literally means with-time, though etymology is misleading here, since Saussure's term describes an atemporal linguistics, linguistics which proceeds without time, which abstracts away from the effects of the ages and studies language at a given, frozen moment."
(Randy Allen Harris, The Linguistic Wars. Oxford Univ. Press, 1993)
- "Most of Saussure's successors accepted the 'synchronic-diachronic' distinction, which still survives robustly in twenty-first-century linguistics. In practice, what this means is that it is accounted a violation of principle or linguistic method to include in the same synchronic analysis evidence related to diachronically different states. So, for example, citing Shakespearean forms would be regarded as inadmissible in support of, say, an analysis of the grammar of Dickens. Saussure is particularly severe in his strictures upon linguists who conflate synchronic and diachronic facts."
(Roy Harris, "Linguists After Saussure." The Routledge Companion to Semiotics and Linguistics, ed. by Paul Cobley. Routledge, 2001)
- Diachronic Linguistics and Historical Linguistics
"For many scholars who would describe their field as 'historical linguistics,' one legitimate target of research involves a focus not on change(s) over time but on the synchronic grammatical systems of earlier language stages. This practice can be called (not unrevealingly) 'old-time synchrony,' and it has made its mark in the form of numerous studies providing synchronic analyses of particular syntactic constructions, word-formation processes, (morpho)phonological alternations, and the like for individual earlier (pre-modern or at least early modern) stages of languages. . . . Gaining as much synchronic information as possible about an earlier stage of a language must surely be viewed as a necessary prerequisite for doing serious work on the diachronic development of a language . . .. Nonetheless, pursuing the synchrony of earlier language states solely for the sake of (synchronic) theory-building . . ., as worthy a goal as it may be, does not count as doing historical linguistics in the literally dia-chronic (through-time) sense that we wish to develop here. At least in a technical sense, then, diachronic linguistics and historical linguistics are not synonymous, because only the latter includes research on 'old-time synchrony' for its own sake, without any focus on language change."
(Richard D. Janda and Brian D. Joseph, "On Language, Change, and Language Change." The Handbook of Historical Linguistics, ed. by B. D. Joseph and R. D. Janda. Blackwell, 2003)