Source : Webster's Revised Unabridged Dictionary (1913)

Decasyllabic \Dec`a*syl*lab"ic\, a. [Pref. deca- + syllabic: cf.
   F. d['e]casyllabique, d['e]casyllable.]
   Having, or consisting of, ten syllables.

Source : WordNet®

decasyllabic
     adj : having or consisting of lines of ten syllables