Source : Webster's Revised Unabridged Dictionary (1913)

Octosyllabic \Oc`to*syl*lab"ic\, Octosyllabical
\Oc`to*syl*lab"ic*al\, a. [L. octosyllabus. See {Octo-}, and
   {Syllable}.]
   Consisting of or containing eight syllables.

Source : WordNet®

octosyllabic
     adj 1: having or consisting of line of eight syllables;
            "octosyllabic verse"
     2: having or characterized by or consisting of eight syllables