| four-dimensional generalization of the three-dimensional Laplacian operator
where is four-nabla, is speed of light in vacuum, and is local time
Note 1 to entry: D’Alembertian operator is useful in STR. In general theory of relativity (GTR) for non-flat space-time, a more sophisticated method is used.
Note 2 to entry: The d'Alembertian operator is a Lorentz scalar.
Note 3 to entry: In the International System of Quantities, the dimension of d'Alembertian operator is .