span·drel also span·dril 
(spăndrəl)
(spăndrəl)n.
1. The roughly triangular space between the left or right exterior curve of an arch and the rectangular framework surrounding it.
2. The space between two arches and a horizontal molding or cornice above them.
[Middle English spaundrell, probably from spandre, space between supporting timbers, from Anglo-Norman spaundre, from spandre, to spread out, from Latin expandere; see EXPAND.]
spandrel