Source : Webster's Revised Unabridged Dictionary (1913)

Gradus \Gra"dus\, n. [From L. gradus ad Parnassum a step to
   Parnassus.]
   A dictionary of prosody, designed as an aid in writing Greek
   or Latin poetry.

         He set to work . . . without gradus or other help. --T.
                                                  Hughes.