eponym is one for whom or which something is or is believed to be named — and, in a second recorded sense, the derived name itself when it comes from that person or thing.
The word runs in two directions, which is the main source of confusion around it. Alois Alzheimer is the eponym of Alzheimer's disease, and the Earl of Sandwich is the eponym of the sandwich. But dictionaries also record the reverse: the name that results — Alzheimer's disease, sandwich, watt, boycott — is itself called an eponym. The adjective eponymous inherits the same ambiguity, describing Henry Ford as the eponymous founder of Ford Motor Company and equally describing a band's eponymous debut album. Neither direction is an error; context does the disambiguating.
The word entered English in the nineteenth century from the Ancient Greek epōnymos, "given as a name" or "giving one's name to something," built from epi ("upon") and onoma ("name") — the same root behind pseudonym, synonym, and anonymous. In classical usage an epōnymos was a legendary founder whose name a tribe or city carried, and later a magistrate whose name designated the year of his tenure.
An eponym is not a 📝pseudonym, which substitutes a fictitious name for a real one, nor a 📝portmanteau, which fuses two existing words into a new one. It is also not a namesake: a namesake merely shares a name with someone, while an eponym is the source of one. 📝Macgyverism, taken from the television character MacGyver, is a working example of the move in its second sense — a proper noun worn down into an ordinary word for the thing it named.