Source : Webster's Revised Unabridged Dictionary (1913)

Drumhead \Drum"head`\, n.
   1. The parchment or skin stretched over one end of a drum.

   2. The top of a capstan which is pierced with sockets for
      levers used in turning it. See Illust. of {Capstan}.

   {Drumhead court-martial} (Mil.), a summary court-martial
      called to try offenses on the battlefield or the line of
      march, when, sometimes, a drumhead has to do service as a
      writing table.

Source : WordNet®

drumhead court-martial
     n : a military court convened to hear urgent charges of offences
         committed in action