Foil
# 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
