Tracery. The ornamental intersecting work in the upper part of a window, screen, or panel, or used decoratively in blank arches and vaults. The earliest use of the term so far traced is in Sir Christopher Wren, the medieval word being form-pieces or forms.
-- John Fleming, Hugh Honour & Nikolaus Pevsner, 1991
The Penguin Dictionary of Architecture
Penguin Books, London, England