2. A thin leaf of sheet copper silvered and burnished, and afterwards coated with transparent colours mixed with isinglass; employed by jewelers to give colour or brilliancy to pastes and inferior stones.
3. Anything that serves by contrast of colour 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)
5. 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.