Written in the notation of partial derivatives, the d'Alembertian 
in a flat spacetime is defined by
 |
where 
is the speed of light.
The operator usually called the d'Alembertian is also the Laplacian on a flat manifold of Lorentzian signature.
d'Alembertian
See also
Gradient Four-Vector, Harmonic Coordinates, Laplacian, Wave EquationExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "d'Alembertian." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/dAlembertian.html