Null vector

In mathematics, given a vector space X with an associated quadratic form q, written (X, q), a null vector or isotropic vector is an element x of X for which q(x) = 0.
In the theory of real bilinear forms, definite quadratic forms and isotropic quadratic forms are distinct. They are distinguished in that only for the latter there exists a nonzero null vector. Where such a vector exists, (X, q) is called a pseudo-Euclidean space.