Source : Webster's Revised Unabridged Dictionary (1913)

Rabbi \Rab"bi\, n.; pl. {Rabbis}or {Rabbies}. [L., fr. Gr. ?,
   Heb. rab[=i] my master, from rab master, lord, teacher, akin
   to Ar. rabb.]
   Master; lord; teacher; -- a Jewish title of respect or honor
   for a teacher or doctor of the law. ``The gravest rabbies.''
   --Milton.

         Be not ye called Rabbi, for one is your Master, even
         Christ, and all ye are brethren.         --Matt. xxiii.
                                                  8.

Source : WordNet®

rabbi
     n 1: spiritual leader of a Jewish congregation; qualified to
          expound and apply Jewish law
     2: a Hebrew title of respect for a Jewish scholar or teacher