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 (narrative), a character who contrasts with another character of a narrative work
  • "Foil" (song), "Weird Al" Yankovic's parody of Lorde's song "Royals"
  • Foil (film), a 2023 sci-fi comedy film
  • 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