Source : Webster's Revised Unabridged Dictionary (1913)

Girder \Gird"er\, n. [From {Gird} to sneer at.]
   One who girds; a satirist.

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

   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}.

   {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.

Source : WordNet®

girder
     n : a beam made usually of steel; a main support in a structure