Tympan

n.
  1. A drum.
  2. (Arch.) A panel; a tympanum.
  3. (Print.) A frame covered with parchment or cloth, on which the blank sheets are put, in order to be laid on the form to be impressed.
    4. Tympan sheet (Print.),
    a sheet of paper of the same size as that to be printed, pasted on the tympan, and serving as a guide in laying the sheets evenly for printing.