1 [noncount] : a very thin and light sheet of metal
Cover the dish with aluminum foil.
kitchen foil [=foil used to wrap or cover food]
2 [count] : someone or something that is different from another person or thing in a useful or appealing way — usually + for or to
He acted as a foil for the comedian. [=he acted very serious to make the comedian seem funnier]
That dark blue dress is a good foil for her blonde hair.
His reserved manner was a perfect foil for/to her bubbly personality.
3 [count] : a sword that has a light, thin blade and that is used in the sport of fencing