The Collaborative International Dictionary of English v.0.48:
Tracery \Tra"cer/y\, n.; pl. Traceries (Arch.)
1. 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.
2. 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
[Webster 1913 Suppl.]