The Collaborative International Dictionary of English v.0.48:

Synecdoche \Syn*ec"do*che\ (s[i^]n*[e^]k"d[-o]*k[-e]), n. [L.
   synecdoche, Gr. synekdochh`, fr. to receive jointly; sy`n
   with + ? to receive; ? out + ? to receive.] (Rhet.)
   A figure or trope by which a part of a thing is put for the
   whole (as, fifty sail for fifty ships), or the whole for a
   part (as, the smiling year for spring), the species for the
   genus (as, cutthroat for assassin), the genus for the species
   (as, a creature for a man), the name of the material for the
   thing made, etc. --Bain.
   [1913 Webster]

WordNet (r) 3.0 (2006):

synecdoche
    n 1: substituting a more inclusive term for a less inclusive one
         or vice versa