An eponym is a person or thing, whether real or fictional, after which a particular place, tribe, era, discovery, or other item is named or thought to be named. For example, Léon Theremin is the eponym of the theremin; Louis Braille is the eponym of the Braille word system created by him for use by the blind. Eponyms are aspects of etymology.
A synonym of "eponym" is namegiver. Someone who (or something that) is referred to with the adjective of eponymous is the eponym of something. An example is: "Léon Theremin, the eponymous inventor of the theremin."
An etiological myth can be a "reverse eponym" in the sense that a legendary character is invented in order to explain a term, such as the nymph Pirene (mythology), who according to myth was turned into Pirene's Fountain.