foil 2 
(foil)
(foil)n.
1. Metal that has been formed into a thin, flexible sheet: aluminum foil.
2. A thin layer of polished metal placed under a displayed gem to lend it brilliance.
3. One that stands in contrast to and emphasizes the distinctive characteristics of another: "I am resolved my husband shall not be a rival, but a foil to me" (Charlotte Brontë).
4. The reflective metal coating on the back of a glass mirror.
5. Architecture A curvilinear, often lobelike figure or space formed between the cusps of intersecting arcs, found especially in Gothic tracery and Moorish ornament.
6.
a. An airfoil.
b. Nautical A hydrofoil.
tr.v. foiled, foil·ing, foils
1. To cover or back with foil.
2. To wrap (strands of hair) in pieces of foil in order to isolate them after bleach or color has been applied.
3. To set off by contrast.
[Middle English, from Old French foille, from Latin folia, pl. of folium, leaf; see bhel-3 in the Appendix of Indo-European roots.]