1 [mass noun] metal hammered or rolled into a thin flexible sheet, used chiefly for covering or wrapping food:aluminium foil
2a person or thing that contrasts with and so emphasizes and enhances the qualities of another:his white cravat was a perfect foil for his bronzed features
a thin leaf of metal placed under a precious stone to increase its brilliance.
3 Architecture a leaf-shaped curve formed by the cusping of an arch or circle.
Origin:
Middle English: via Old French from Latin folium 'leaf'
