An Etymological Dictionary of Astronomy and Astrophysics

The partial differential equation

∂²U / ∂²x + ∂²U / ∂²y + ∂²U / ∂²z = (1/c²) ∂²U / ∂²t
or its counterparts in one or two dimensions or in other coordinates,
the solution of which represents the propagation of displacementU as waves with velocity c.

See also: → wave; → equation.