Glottochronology (from Attic Greek γλῶττα “tongue, language” and χρóνος “time”) is that part of lexicostatistics dealing with the chronological relationship between languages.

The idea was developed by Morris Swadesh under two assumptions: first that there exists a relatively stable "basic vocabulary" (referred to as "Swadesh lists") in all languages of the world, and second that any replacements happen in a way analogical to that in radioactive decay in constant percentages per time elapsed. Meanwhile there exist many different methods, partly extensions of the Swadesh method, now more and more under the biological assumptions of replacements in genes. However, Swadesh's technique is so well known that, for many people, 'glottochronology' refers to it alone.