foil

An acronym for the algorithm for multiplying two binomials.

To apply the FOIL algorithm to.

A very thin sheet of metal or plastic.

Thin aluminium/aluminum (or, formerly, tin) used for wrapping food.

wrap the sandwich up in foil

A thin layer of metal put between a jewel and its setting to make it seem more brilliant.

In literature, theatre/theater, etc., a character who helps emphasize the traits of the main character and who usually acts as an opponent or antagonist, but can also serve as the sidekick of the protagonist.

Whereas the female invests heavily in the wake of conception, the male invests heavily prior to conception. Hitherto, the male has been regarded as the mere foil to the heavy investment in reproduction that renders the female the logjam (the limiting factor) in reproduction – reproduction overall is expandable by increasing the number of females, not by increasing the number of males.

As Puss comes to realize his legendary status is not a substitute for interpersonal connection, his interactions with Kitty and the therapy dog start to take on a surprising amount of weight, while Goldilocks serves as a well-realized foil who has more in common with Puss that is at first apparent.

Anything that acts by contrast to emphasise the characteristics of something.

As she a black silk cap on him begun / To set, for foil of his milk-white to serve.

1725-1726, William Broome, The Odyssey Hector has also a foil to set regard