This is not what you make an appointment at the spa for – that’s exfoliation. No, this happy little word is related to foliage – i.e., trees and leaves – and actual foils (and not the kind you wrap your leftovers in). In the world of the decorative arts and architecture, a foil is a lobe or leaf-shaped curve formed by the cusping of a circle or an arch. Very Gothic, but also Moorish. The number of foils involved is indicated by a prefix, e.g. trefoil (3), quatrefoil (4), cinquefoil (5), multifoil, etc. So it makes sense that foliated ornamentation features foils and/or representations of foliage. In architecture, the application commonly is on an archway, window or other opening; in the decorative arts, you can apply it to just about anything.
See also: cinquefoil