FOIL

n. 1 a metal hammered or rolled into a thin sheet (tin foil). b a sheet of this, or of tin amalgam, attached to mirror glass as a reflector. c a leaf of foil placed under a precious stone etc. to brighten or colour it. 2 a person or thing that enhances the qualities of another by contrast. 3 Archit. a leaf-shaped curve formed by the cusping of an arch or circle. [ME f. OF f. L folium leaf, and f. OF foille f. L folia (pl.)]