The Collaborative International Dictionary of English v.0.48:

Girder \Gird"er\, n. [From Gird to encircle.]
   [1913 Webster]
   1. One who, or that which, girds.
      [1913 Webster]

   2. (Arch. & Engin.) A main beam; a stright, horizontal beam
      to span an opening or carry weight, such as ends of floor
      beams, etc.; hence, a framed or built-up member
      discharging the same office, technically called a compound
      girder. See Illusts. of Frame, and Doubleframed floor,
      under Double.
      [1913 Webster]

   Bowstring girder, Box girder, etc. See under Bowstring,
      Box, etc.

   Girder bridge. See under Bridge.

   Lattice girder, a girder consisting of longitudinal bars
      united by diagonal crossing bars.

   Half-lattice girder, a girder consisting of horizontal
      upper and lower bars connected by a series of diagonal
      bars sloping alternately in opposite directions so as to
      divide the space between the bars into a series of
      triangles. --Knight.

   Sandwich girder, a girder consisting of two parallel wooden
      beams, between which is an iron plate, the whole clamped
      together by iron bolts.
      [1913 Webster]