Source : Webster's Revised Unabridged Dictionary (1913)

Copyholder \Cop"y*hold`er\, n.
   1. (Eng. Law) One possessed of land in copyhold.

   2. (print.)
      (a) A device for holding copy for a compositor.
      (b) One who reads copy to a proof reader.

Source : WordNet®

copyholder
     n : mechanical device used in printing; holds the copy for the
         compositor