The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

         He set to work . . . without gradus or other help. --T.
                                                  Hughes.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

GRADUS. This is a Latin word, literally signifying a step; figuratively it
is used to designate a person in the ascending or descending line, in
genealogy; a degree.