1. n. [Geophysics]

A mathematical expression to represent wave displacement and wave velocity (V) as functions of space (x, y, z) and time (t).

∇²ψ = ∂²ψ/∂x² + ∂²ψ/∂y² + ∂²ψ/∂z² = (1/V²) ∂²ψ/∂t²,

where
ψ = wave displacement
V = wave velocity
x, y and z = space coordinates
t = time.

See related terms: back-propagation,  diffusion equation,  Kirchhoff equation