The Collaborative International Dictionary of English v.0.48:
Foil \Foil\, n. [OE. foil leaf, OF. foil, fuil, fueil, foille,
fueille, F. feuille, fr. L. folium, pl. folia; akin to Gr. ?,
and perh. to E. blade. Cf. Foliage, Folio.]
1. A leaf or very thin sheet of metal; as, brass foil; tin
foil; gold foil.
2. (Jewelry) A thin leaf of sheet copper silvered and
burnished, and afterwards coated with transparent colors
mixed with isinglass; -- employed by jewelers to give
color or brilliancy to pastes and inferior stones. --Ure.
3. Anything that serves by contrast of color or quality to
adorn or set off another thing to advantage.
As she a black silk cap on him began
To set, for foil of his milk-white to serve. --Sir
Hector has a foil to set him off. --Broome.
4. A thin coat of tin, with quicksilver, laid on the back of
a looking-glass, to cause reflection.
5. (Arch.) The space between the cusps in Gothic
architecture; a rounded or leaflike ornament, in windows,
niches, etc. A group of foils is called trefoil,
quatrefoil, quinquefoil, etc., according to the number of
arcs of which it is composed.
Foil stone, an imitation of a jewel or precious stone.