In special relativity, a four-vector (or 4-vector, sometimes Lorentz vector) is an element of a four-dimensional vector space object with four components, which transform under Lorentz transformations with respect to a change of basis. Its magnitude is determined by an indefinite quadratic form, the preservation of which defines the Lorentz transformations, which include spatial rotations and boosts (a change by a constant velocity to another reference frame).
Four-vectors describe, for instance, position x in spacetime modeled as Minkowski space, a particle's four-momentum p, the amplitude of the electromagnetic four-potential A(x) at a point x in spacetime, and the elements of the subspace spanned by the gamma matrices inside the Dirac algebra.