Symbol □ (sometimes printed □2). An operator that is the analogue of the Laplace operator in four-dimensional Minkowski space–time, i.e. □ = ∂2/∂x2 + ∂2/∂y2 + ∂2/∂z2 − (1/c2) ∂2/∂t2,
□ = ∂2/∂x2 + ∂2/∂y2 + ∂2/∂z2 − (1/c2) ∂2/∂t2,
where c is the speed of light. The D'Alembertian is very useful in the special theory of relativity.