The Collaborative International Dictionary of English v.0.48:

Eponym \Ep"o*nym\, Eponyme \Ep"o*nyme\, n. [Cf. F. ['e]ponyme.
   See Eponymous.]
   1. The hypothetical individual who is assumed as the person
      from whom any race, city, etc., took its name; as, Hellen
      is an eponym of the Hellenes.
      [1913 Webster]

   2. A name, as of a people, country, and the like, derived
      from that of an individual.
      [1913 Webster]

WordNet (r) 3.0 (2006):

eponym
    n 1: the person for whom something is named; "Constantine I is
         the eponym for Constantinople"
    2: the name derived from a person (real or imaginary); "Down's
       syndrome is an eponym for the English physician John Down"