In a relativistic theory of physics, a Lorentz scalar is a scalar expression whose value is invariant under any Lorentz transformation. A Lorentz scalar may be generated from, for example, the scalar product of vectors, or by contracting a tensor. While the components of the contracted quantities may change under Lorentz transformations, the Lorentz scalars remain unchanged.
A simple Lorentz scalar in Minkowski spacetime is the spacetime distance ("length" of their difference) of two fixed events in spacetime. While the "position"-4-vectors of the events change between different inertial frames, their spacetime distance remains invariant under the corresponding Lorentz transformation. Other examples of Lorentz scalars are the "length" of a 4-velocity (see below), or the Ricci curvature at a point in spacetime in general relativity, which is a contraction of the Riemann curvature tensor.