Source : Webster's Revised Unabridged Dictionary (1913)

Straightedge \Straight"edge`\, n.
   A board, or piece of wood or metal, having one edge perfectly
   straight, -- used to ascertain whether a line is straight or
   a surface even, and for drawing straight lines.

Source : WordNet®

straightedge
     n : hand tool consisting of a flat rigid rectangular bar (metal
         or wood) that can be used to draw straight lines (or test
         their straightness)