The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. The barrel of a wooden pump.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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, penstock, head gate,
         water gate]
    2: conduit that carries a rapid flow of water controlled by a
       sluicegate [syn: sluice, sluiceway, penstock]