The Collaborative International Dictionary of English v.0.48:

Joist \Joist\ (joist), n. [OE. giste, OF. giste, F. g[^i]te, fr.
   gesir to lie, F. g['e]sir. See Gist.] (Arch.)
   A piece of timber laid horizontally, or nearly so, to which
   the planks of the floor, or the laths or furring strips of a
   ceiling, are nailed; -- called, according to its position or
   use, binding joist, bridging joist, ceiling joist,
   trimming joist, etc. See Illust. of Double-framed floor,
   under Double, a.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Bridgeing \Bridge"ing\, n. (Arch.)
   The system of bracing used between floor or other timbers to
   distribute the weight.
   [1913 Webster]

   Bridging joist. Same as Binding joist.
      [1913 Webster]