In fiction, a foil is a character who contrasts with another character - usually the protagonist— to highlight particular qualities of the other character. In some cases, a subplot can be used as a foil to the main plot. In elementary algebra, FOIL is a mnemonic for the standard method of multiplying two binomials—hence the method may be referred to as the FOIL method. 2 : a thin piece of material (such as metal) put under an inferior or paste stone to add color or brilliance. 3 : someone or something that serves as a contrast to another. acted as a foil for a comedian. a thin layer of metal placed under a gem in a closed setting to improve its color or brilliancy. a person or thing that makes another seem better by contrast: The straight man was an able foil to the comic. Architecture. an arc or a rounded space between cusps, as in the tracery of a window or other ornamentation. From Middle English foil, foille, from Old French fueille ('plant leaf'), from Latin folia, the plural of folium, mistaken as a singular feminine. Doublet of folio. One that stands in contrast to and emphasizes the distinctive characteristics of another: 'I am resolved my husband shall not be a rival, but a foil to me' (Charlotte . Made in the USA since 1947, Reynolds Wrap Aluminum Foil has trusted strength and quality you can count on for cooking delicious meals with easy clean-up. You can serve as a foil to someone if you show them to be better than you by contrast. If you can't dance but your friend Lisa can, you can be a foil to Lisa's grace . Jewelers often put shiny metal foil underneath a gem to make the stone shine brighter. A literary foil serves a similar purpose. This character highlights someone .