The Collaborative International Dictionary of English v.0.48:

Eponymy \E*pon"y*my\, n. [Gr. ? a surname given after some
   person or thing.]
   The derivation of the name of a race, tribe, etc., from that
   of a fabulous hero, progenitor, etc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

eponymy
    n 1: the derivation of a general name from that of a famous
         person