A Perturbation to a Structure that moves.

In Mechanics and Condensed matter physics, it refers to Displacement of the Atoms relative to the equilibrium positions in the Crystal lattice in a crystalline material. More generally, it refers to variations in the particle density relative to the mean density.

When the dispersion relation of the pressure waves is linear, the waves are called Sound.

Mathematically, it is often described via a Wave equation. This can be derived as the continuum limit of a system of Coupled oscillators.