A foil is a type of practice weapon used in fencing. See foil (sword)
Foil can also refer to a hydrofoil, a type of boat.
A related yet different meaning of foil is airfoil, a type of wing or blade.
Foil can also be a short form of the longer noun aluminium foil, a type of wrapping for food, as in "I've wrapped the sandwiches in foil".
A foil is also a type of character in literature. See foil (literature)