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