foil

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Metal that has been formed into a thin, flexible sheet.
• noun A thin layer of polished metal placed under a displayed gem to lend it brilliance.
• noun One that stands in contrast to and emphasizes the distinctive characteristics of another.
• noun The reflective metal coating on the back of a glass mirror.
• noun Architecture A curvilinear, often lobelike figure or space formed between the cusps of intersecting arcs, found especially in Gothic tracery and Moorish ornament.
• noun An airfoil.
• noun Nautical A hydrofoil.
• transitive verb To cover or back with foil.
• transitive verb To wrap (strands of hair) in pieces of foil in order to isolate them after bleach or color has been applied.
• transitive verb To set off by contrast.
• noun A fencing sword having a usually circular guard and a thin, flexible four-sided blade with a button on the tip to prevent injury.
• noun The art or sport of fencing with such a sword.
• transitive verb To prevent from being successful; thwart.
• transitive verb To obscure or confuse (a trail or scent) so as to evade pursuers.
• noun A repulse; a setback.
• noun The trail or scent of an animal.

from The Century Dictionary.

• To defile: same as file, foul.
• noun The track or trail of game when pursued.
• noun Defeat; frustration; failure when on the point of achievement.
• noun In wrestling, a partial fall; a fall not complete according to the rules.
• noun A leaf, as of a plant.
• noun A metallic substance formed into very thin sheets by rolling and hammering: as, gold, tin, or lead foil.
• noun In jewelry, a thin leaf of metal placed under a precious stone to change its color, or to give it more color in case of its being inferior in that respect, or to give it additional luster by the reflection of light from the surface of the metal.
• noun Leaf-metal placed behind translucent enamel for the same purpose as that used for precious stones. (See def. 3.) In this sense often called paillon (which see). Hence Anything of a different color or of different qualities which serves to adorn or set off another thing to advantage; that which, by comparison or contrast, sets off or shows more conspicuously the superiority of something else.
• noun An amalgam of tin with quicksilver laid on one side of a sheet of glass to produce a reflecting surface in making a mirror.
• noun In medieval architecture, a small arc in the tracery of a window, panel, etc., which is said to be trefoiled, quatrefoiled, cinquefoiled, multifoiled, etc., according to the number of arcs which it contains.
• To trample upon; tread under foot.
• To blunt; dull; deaden: as, to foil the scent in a chase.
• To frustrate; baffle; mislead; render vain or nugatory, as an effort or attempt; thwart; balk: as, the enemy was foiled in his attempt to pass the river.
• This your courtesy
• Synonyms Thwart, Baffle, etc. See frustrate.
• noun A bated or blunted sword used in fencing-practice and friendly contests; now, usually, an implement used in fencing-schools, for small-sword practice only.
• In hunting, of an animal, to retrace its own track for the purpose of baffling the hounds; ‘run the foil.’ See foil, n.

from the GNU version of the Collaborative International Dictionary of English.

• transitive verb obsolete To defile; to soil.
• noun Failure of success when on the point of attainment; defeat; frustration; miscarriage.
• noun A blunt weapon used in fencing, resembling a smallsword in the main, but usually lighter and having a button at the point.
• noun The track or trail of an animal.
• noun to lead astray; to puzzle; -- alluding to the habits of some animals of running back over the same track to mislead their pursuers.
• transitive verb To tread under foot; to trample.
• transitive verb To render (an effort or attempt) vain or nugatory; to baffle; to outwit; to balk; to frustrate; to defeat.
• transitive verb To blunt; to dull; to spoil.
• noun A leaf or very thin sheet of metal
• noun (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.
• noun Anything that serves by contrast of color or quality to adorn or set off another thing to advantage.
• noun A thin coat of tin, with quicksilver, laid on the back of a looking-glass, to cause reflection.
• noun (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.
• noun an imitation of a jewel or precious stone.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun hunting The track of an animal.
• verb To prevent (something) from being accomplished.