In mathematics, a quadratic form is a homogeneous polynomial of degree two in a number of variables. For example,

is a quadratic form in the variables x and y.

Quadratic forms occupy a central place in various branches of mathematics, including number theory, linear algebra, group theory (orthogonal group), differential geometry (Riemannian metric), differential topology (intersection forms of four-manifolds), and Lie theory (the Killing form).