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
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