An Etymological Dictionary of Astronomy and Astrophysics

A second order, → partial differential operator in space-time, defined as: &#9643² = ∂²/∂x² + ∂²/∂y² + ∂²/∂z² - (1/c²)∂²/∂t², or &#9643² = ∇² - (1/c²)(∂²/∂t²), where ∇² is the → Laplacian and c is the → speed of light. This operator is the square of the → four-dimensional operator 
&#9643, which is Lorentz invariant.

See also: → d’Alembert’s principle; → operator.