autonomasy

from The Collaborative International Dictionary of English v.0.48

Autonomasy \Au`to*nom"a*sy\, n. [Auto- + Gr. ? a name, fr. ? a
   name; or for E. antonomasia.] (Rhet.)
   The use of a word of common or general signification for the
   name of a particular thing; as, "He has gone to town," for,
   "He has gone to London."
   [1913 Webster]