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.
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   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.
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   3. Anything that serves by contrast of color or quality to
      adorn or set off another thing to advantage.
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            As she a black silk cap on him began
            To set, for foil of his milk-white to serve. --Sir
                                                  P. Sidney.
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            Hector has a foil to set him off.     --Broome.
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   4. A thin coat of tin, with quicksilver, laid on the back of
      a looking-glass, to cause reflection.
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   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.
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   Foil stone, an imitation of a jewel or precious stone.
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