The Collaborative International Dictionary of English v.0.48:

Cablegram \Ca"ble*gram`\ (k[=a]"b'l*gr[a^]m`), n. [Cable, n. +
   Gr. gra`mma a writing, a letter.]
   A message sent by a submarine telegraphic cable.

   Note: [A recent hybrid, sometimes found in the newspapers.]
         [1913 Webster]

WordNet (r) 3.0 (2006):

cablegram
    n 1: a telegram sent abroad [syn: cable, cablegram,
         overseas telegram]