picture consisting of a positive photograph or drawing on a transparent base; viewed with a projector
"The teacher displayed a foil of the historical event onto the wall using an overhead projector so the entire class could see the details."
a light slender flexible sword tipped by a button
"The fencer raised his foil to parry the opponent's thrust before executing a quick lunge."
A very thin sheet of metal.
"The fox's long, bushy tail acted as a foil to its slender legs when it darted through the underbrush."
Failure when on the point of attainment; defeat; frustration; miscarriage.
The track of an animal.
In plain English: A foil is a path or trail left by something moving through snow or other soft ground.
"The deer left a clear foil in the fresh snow."
To apply the FOIL algorithm to.
"The careless visitor managed to foil the pristine white carpet with muddy boots."
To cover or wrap with foil.
To prevent (something) from being accomplished.
To expand a product of two or more algebraic expressions, typically binomials.
To defile; to soil.
In plain English: To foil means to damage something's reputation or purity.
"The muddy boots threatened to foil the pristine white carpet."
Usage: Use "foil" as a verb to mean preventing someone or something from succeeding, such as foiling an enemy's plan. Do not confuse this with its algebraic meaning of expanding expressions, which is unrelated to everyday usage.
An acronym for the algorithm for multiplying two binomials.
"The math teacher asked if anyone knew what FOIL stands for, which is the method for multiplying two binomials."
The word "foil" is not an acronym; it originates from the Old French word foile, meaning a thin sheet of metal used to protect or decorate other surfaces. Over time, its meaning expanded in English to include anything that serves as a contrast or impediment to something else.