Foil consists of sheets of metal as thin as paper. It is used to wrap food in.

If you foil someone's plan or attempt to do something, for example to commit a crime, you succeed in stopping them from doing what they want.

If you refer to one thing or person as a foil for another, you approve of the fact that they contrast with each other and go well together, often in a way that makes the second thing or person seem better or less harmful.

A foil is a thin light sword used in fencing, which has a button on its tip to prevent injury.