${\displaystyle \square =◊\cdot ◊=\Delta -\frac{{\partial }^2}{c_0^2\partial t^2}}$

where $◊$ is four-nabla, $c_0$ is speed of light in vacuum, and $t$ 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 ${\mathrm{L}}^{-2}$.