“Diachronic” is a term indicating “change across time” which was much used in linguistics during the 1960s and from there entered critical theory. The term stands in contrast with the “synchronic” which may mean “that which is the same throughout time” or “that which is without time”. For example, the synchronic axis in language use may be conceived of as the entire corpus of grammar and the lexicon which enables an utterance to be made, whilst the utterance is produced in time and is therefore diachronic. A synchronic study of a language attempts to establish the system as a functional whole, whereas a diachronic study considers its historical evolution.
Editors. "Synchronic, Synchrony". The Literary Encyclopedia. First published 24 July 2009
[http://www.litencyc.com/php/stopics.php?rec=true&UID=1085, accessed 30 September 2016.]