A hydronym (from Greekhudor, "water" and onuma, "name") is a proper name of a body of water. Hydronymy is the study of hydronyms and of how bodies of water receive their names and how they are transmitted through history. It can apply to rivers, lakes, even oceanic elements.
Like most toponyms, as linguistic items, hydronyms are very conservative, with successor peoples often retaining the name given a body of water. For example, Mississippi has passed from Native Americans to contemporary Americans. Often a given body of water will have several entirely different names given to it by different peoples. For example, Vltava and Moldau are the Czech and German names, respectively, for the same river in central Europe.
Among the Indo-European languages, hydronyms from various languages can all share a common etymon: The Danube, Don, Dniester, Dnieper and Donets rivers all contain the Scythian name for "river" (cf. don "river, water" in modern Ossetic).
