Plane wave

A wave in which each acoustic parameter (particle displacement, density, pressure, etc.) has a constant value on any specified plane that is perpendicular to the direction of wave propagation -- i.e., each acoustic parameter depends only on one spatial coordinate, where the coordinate axis is in the direction of the wave propagation. Kinsler, p. 98, 107

At great distances from the source of a wave disturbance, all waves will become approximately plane. Kinsler, p. 98