1. [n]  a light slender flexible sword tipped by a button
2. [n]  a piece of thin and flexible sheet metal; "the photographic film was wrapped in foil"
3. [n]  picture consisting of a positive photograph or drawing on a transparent base; viewed with a projector
4. [n]  a device consisting of a flat or curved piece (as a metal plate) so that its surface reacts to the water it is passing through; "the fins of a fish act as hydrofoils"
5. [n]  anything that serves by contrast to call attention to another thing's good qualities; "pretty girls like plain friends as foils"
6. [v]  cover or back with foil; "foil mirrors"
7. [v]  hinder or prevent (the efforts, plans, or desires) of; "What ultimately frustrated every challenger was Ruth's amazing September surge"; "foil your opponent"
8. [v]  enhance by contrast; "In this picture, the figures are foiled against the background"

1. \Foil\ (foil), v. t. [imp. & p. p. {Foiled} (foild); p. pr.
   & vb. n. {Foiling}.] [F. fouler to tread or trample under
   one's feet, to press, oppress. See {Full}, v. t.]
   1. To tread under foot; to trample.
            King Richard . . . caused the ensigns of Leopold to
            be pulled down and foiled under foot. --Knoless.
            Whom he did all to pieces breake and foyle, In
            filthy durt, and left so in the loathely soyle.
                                                  --Spenser.
   2. To render (an effort or attempt) vain or nugatory; to
      baffle; to outwit; to balk; to frustrate; to defeat.
            And by ? mortal man at length am foiled. --Dryden.
            Her long locks that foil the painter's power.
                                                  --Byron.
   3. To blunt; to dull; to spoil; as, to foil the scent in
      chase. --Addison.
   

2. \Foil\, v. t. [See 6th {File}.]
   To defile; to soil. [Obs.]
   

3. \Foil\, n.
   1. Failure of success when on the point of attainment;
      defeat; frustration; miscarriage. --Milton.
            Nor e'er was fate so near a foil.     --Dryden.
   2. A blunt weapon used in fencing, resembling a smallsword in
      the main, but usually lighter and having a button at the
      point.
            Blunt as the fencer's foils, which hit, but hurt
            not.                                  --Shak.
            Isocrates contended with a foil against Demosthenes
            with a word.                          --Mitford.
   3. The track or trail of an animal.
   {To run a foil},to lead astray; to puzzle; -- alluding to the
      habits of some animals of running back over the same track
      to mislead their pursuers. --Brewer.
   

4. \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.
   

File Oriented Interpretive Language. CAI language.

["FOIL - A File Oriented Interpretive Language", J.C. Hesselbart, Proc ACM 23rd National Conf (1968)].