Foil

Foil may refer to:

Materials

 * Foil (metal), a quite thin sheet of metal, usually manufactured with a rolling mill machine
 * Metal leaf, a very thin sheet of decorative metal
 * Aluminium foil, a type of wrapping for food
 * Tin foil, metal foil made of tin, the direct predecessor to aluminium foil
 * Transparency (projection), a thin sheet of transparent flexible material, placed on an overhead projector for display to an audience

Fluid dynamics

 * Foil (fluid mechanics)
 * Airfoil, a foil operating in air
 * Hydrofoil, a foil operating in water
 * Parafoil, a non-rigid airfoil, inflated during use
 * Foil bearing, a type of fluid bearing

Arts and culture

 * Foil (architecture), decorative device derived from cusps of circles
 * Foil stamping, a printmaking technique
 * Foil (fencing), one of the three weapons used in modern fencing
 * Foil (fiction), a subsidiary character who emphasizes the traits of a main character
 * Comedic or comic foil, the straight man in a comedy double act
 * "Foil" (song), "Weird Al" Yankovic's parody of Lorde's song "Royals"

Navigation

 * Hydrofoil, a type of high-powered motorboat that uses underwater foils to lift its hull above the water when moving at high speeds
 * Bruce foil, a foil used on an outrigger to prevent a boat from heeling
 * Centreboard, a movable keel that functions as a foil
 * Foilboard, a surfboard using a hydrofoil

Other uses

 * People in a police lineup
 * First-order inductive learner – a rule-based learning algorithm
 * The FOIL method, a mnemonic in algebra, to expand the product of two first-degree polynomials ("linear factors")
 * FOIL (programming language), either of two now-defunct computer programming languages
 * Forum of Indian Leftists, a political group of Indian intellectuals
 * Freedom of information legislation or Freedom of Information Law (FOIL)
 * Ultrasonic foil (papermaking), a type of high-frequency vibrating foil involved in papermaking
 * Split tally, in ancient financial accounting, the part of a split tally stick given to the recipient in a transaction