Source : Webster's Revised Unabridged Dictionary (1913)

Prorate \Pro*rate"\, v. t. [imp. & p. p. {Prorated}; p. pr. &
   vb. n. {Prorating}.] [From L. pro rata (sc. parte) according
   to a certain part, in proportion.]
   To divide or distribute proportionally; to assess pro rata.
   [U.S.]

Source : WordNet®

prorate
     v 1: make a proportional settlement or distribution
     2: divide or assess proportionally; "The rent was prorated for
        the rest of the month"