plane wave

A plane wave (or plane parallel wave) is a theoretical wave such that the motions/changes occurring are the same across an entire mathematical plane, and the wave is traveling perpendicular to the plane. Contrast this to a sound wave coming from an object a few feet in front of you: each wave peak forms an expanding sphere in all directions from the object, i.e., it is curved. But a sound wave reaching your vicinity from a long distance is nearly a plane wave, showing very little such curvature.

A plane wave is mathematically simpler than waves diverging from something, e.g., as expanding spheres, but a sub-region consisting of a small fraction of such an expanding sphere approximates a plane, and as such a wave progresses, using a plane wave as an approximation of the wave becomes more and more useful. Thus, a wave from a distant source (e.g., light waves from stars) is often modeled as a plane wave to generalize the model and simplify the math. This is analogous to the "plane-parallel atmosphere" simplified model of atmospheres that ignore its curved shape.

A plane wave (if longitudinal) also approximates a wave within a cylindrical musical wind instrument such as a flute or organ pipe, one complicating factor being the effect of friction between the medium (air) and the surrounding inner pipe surface.

Given the relevance of waves to electromagnetism and quantum theory, as well as waves due to general relativity (GR) and the effect of GR on waves, the fact that they can often be usefully modeled as plane waves is extremely helpful.