Source : Webster's Revised Unabridged Dictionary (1913)

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 P.
                                                  Sidney.

            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.