d'Alembert operator

Etymology
Named after (1717–1783), a French mathematician, mechanician, physicist, philosopher, and music theorist.

Noun

 * 1)  A differential operator which may be expressed as $$\partial_\mu \partial^\mu = \sum_{\mu = 0}^3 {\partial \over \partial x^\mu}{\partial \over \partial x_\mu}$$; it is the four-dimensional (Minkowski space) equivalent of the three-dimensional Laplace operator.

Usage notes

 * It may be denoted as $$\Box^2$$ (in analogy with the $$\nabla^2$$ symbol for the Laplacian) or as $$\Box$$ (in analogy with the $$\Delta$$ symbol for the Laplacian).
 * It may also be denoted as $$\partial^2$$.