The Collaborative International Dictionary of English v.0.48:

Metonymy \Me*ton"y*my\ (m[-e]*t[o^]n"[i^]*m[y^]; 277), n. [L.
   metonymia, Gr. metwnymi`a; meta`, indicating change +
   'o`nyma, for 'o`noma a name: cf. F. m['e]tonymie. See
   Name.] (Rhet.)
   A trope in which one word is put for another that suggests
   it; as, we say, a man keeps a good table instead of good
   provisions; we read Virgil, that is, his poems; a man has a
   warm heart, that is, warm affections; a city dweller has no
   wheels, that is, no automobile.
   [1913 Webster]

WordNet (r) 3.0 (2006):

metonymy
    n 1: substituting the name of an attribute or feature for the
         name of the thing itself (as in `they counted heads')