 d'Alembert operator

The d'Alembert operator is the Lorentzian wave operator $\square ={\eta }^{\mu \nu }{\partial }_{\mu }{\partial }_{\nu }$ in flat spacetime, or $\square ={\nabla }^{\mu }{\nabla }_{\mu }$ on scalar fields in curved spacetime.

The laplace operator for Minkowski space.

Can be nicely written with Einstein notation as shown at: Section "d'Alembert operator in Einstein notation".