1. Someone who gives, or is supposed to give, his/her name to a people, place, or institution; e.g. among the Greeks, the heroes who were looked upon as ancestors or founders of tribes or cities. Also in Latin form eponymus.
2. A name from which another name or word is derived; such as, "sandwich" coming from The Earl of Sandwich and "Rome" which is derived from Romulus.
3. The name of a method, test, etc., which is based on the name of a person connected with it in some way.