Source : Webster's Revised Unabridged Dictionary (1913)

Octosyllable \Oc"to*syl`la*ble\, a.
   Octosyllabic.

Octosyllable \Oc"to*syl`la*ble\, n.
   A word of eight syllables.

Source : WordNet®

octosyllable
     n : a verse line having eight syllables or a poem of
         octosyllabic lines