Source : Webster's Revised Unabridged Dictionary (1913)

Tracery \Tra"cer*y\, n.
   A tracing of lines; a system of lines produced by, or as if
   by, tracing, esp. when interweaving or branching out in
   ornamental or graceful figures. ``Knit with curious
   tracery.'' --Burns.

Tracery \Tra"cer/y\, n.; pl. {Traceries} (Arch.)
   Ornamental work with rambled lines. Especially:
   (a) The decorative head of a Gothic window.

   Note: Window tracery is of two sorts, plate tracery and bar
         tracery. Plate tracery, common in Italy, consists of a
         series of ornamental patterns cut through a flat plate
         of stone. Bar tracery is a decorative pattern formed by
         the curves and intersections of the molded bars of the
         mullions. Window tracery is imitated in many decorative
         objects, as panels of wood or metal either pierced or
         in relief. See also Stump tracery under {Stump}, and
         Fan tracery under {Fan}.
   (b) A similar decoration in some styles of vaulting, the ribs
       of the vault giving off the minor bars of which the
       tracery is composed.

Source : WordNet®

tracery
     n : decoration consisting of an open pattern of interlacing ribs