Definition of "hydronym"

Numerous hydronyms of Asia Minor, attested in the historical Anatolian languages and going back to Proto-Anatolian, can be compared to the Indo-European derivational type of hydronyms in *-nth-. […] Another hydronym of Proto-Anatolian origin and having wide Indo-European connections is Alda, a river in Asia Minor […]

1995, Thomas V. Gamkrelidze, Vjačeslav V. Ivanov, “Proto-Indo-European in Space and Time, Based on Linguistic and Culture-historical Data [part 2, chapter 11]”, in Johanna Nichols, transl., edited by Werner Winter, Indo-European and the Indo-Europeans: A Reconstruction and Historical Analysis of a Proto-Language and Proto-Culture (Trends in Linguistics: Studies and Monographs; 80), Berlin, New York, N.Y.: Mouton de Gruyter, page 760

Dutch Zijpe, Zijp (and regional variants) is found in toponyms, e.g. the poulder De/Het Zijpe in the province of North-Holland, the name of the castle Sijpestein (Nieuw-Loosdrecht), […] We are dealing with an old hydronym.

2000, Dirk Boutan, Maarten Kossmann, “On the Etymology of Dutch Zijpe”, in Dirk Boutkan, Arend Quak, editors, Language Contact: Substratum, Superstratum, Adstratum in Germanic Languages (Amsterdamer Beiträge Zur Älteren Germanistik; 54), Amsterdam: Rodopi, page 31

Considering the fact that the Yeneseic languages are historically spoken along almost the entire length of the Yenesei from the Minusinsk basin to the Arctic zone it is curious that no Common Yeneseic name for the river seems to have been preserved in the linguistic material. […] A hydronym that possibly belongs to this context is Kezes, which has been derived from Ket (*)qä-ses 'large water' […]

2012, Juha Janhunen, “Etymological and Ethnohistorical Aspects of the Yenesei”, in Marek Stachowski, editor, Studia Etymologica Cracoviensia, volume 17, Kraków: Jagiellonian University Press, page 73

A river name is a hydronym, a name of a body of water. The whole stock of hydronyms in a district, territory, country, or the like is called its hydronymy.

2016, Svante Strandberg, “River Names”, in Carole Hough, Daria Izdebska, editors, The Oxford Handbook of Names and Naming, Oxford: Oxford University Press, page 105