The Collaborative International Dictionary of English v.0.48:

Flash \Flash\, n. [OE. flasche, flaske; cf. OF. flache, F.
   flaque.]
   1. A pool. [Prov. Eng.] --Haliwell.
      [1913 Webster]

   2. (Engineering) A reservoir and sluiceway beside a navigable
      stream, just above a shoal, so that the stream may pour in
      water as boats pass, and thus bear them over the shoal.
      [1913 Webster]

   Flash wheel (Mech.), a paddle wheel made to revolve in a
      breast or curved water way, by which water is lifted from
      the lower to the higher level.
      [1913 Webster]