1. In medicine, relating to something systematically observed over time in the same subjects throughout as opposed to synchronic or cross-sectional: In his studies, John was involved in diachronic research regarding patients only where there was strict stability of all elements.
2. Pertaining to a linguistic study concerned with the historical development of a language: Since Jane was interested in languages, she decided to learn more about the diachronic features of German throughout time.
3. In archaeology, denoting actions or things which occur over time: In Sam's diachronic research, he classified the artifacts he had found in his region as they developed or changed throughout sequential periods of time.