Source : Webster's Revised Unabridged Dictionary (1913)

Penstock \Pen"stock\, n. [Etymol. uncertain; perh. fr. pen an
   inclosure + stock.]
   1. A close conduit or pipe for conducting water, as, to a
      water wheel, or for emptying a pond, or for domestic uses.

   2. The barrel of a wooden pump.

Source : WordNet®

penstock
     n 1: regulator consisting of a valve or gate that controls the
          rate of water flow through a sluice [syn: {sluicegate},
          {sluice valve}, {floodgate}, {head gate}, {water gate}]
     2: conduit that carries a rapid flow of water controlled by a
        sluicegate [syn: {sluice}, {sluiceway}]