Decorative Panel Either Side of an Arch
Puzzle 18 of Periodic Table Medium Pack
Question is: Decorative Panel Either Side of an Arch
and the answer is: SPANDREL
Word Meaning:
SPANDREL — (architecture) The space (often more or less triangular) between the outer curve of an arch (the extrados) and a straight-sided figure that bounds it; the space between two contiguous arches and a straight feature above them.
Definitions from Wiktionary.