The Collaborative International Dictionary of English v.0.48:

Panhellenium \Pan`hel*le"ni*um\, n. [NL., from Gr.
   "Panellh`nion; pa^s, pa^n, all + "E`llhnes the Greeks.] (Gr.
   Antiq.)
   An assembly or association of Greeks from all the states of
   Greece.
   [1913 Webster]