The Brunt-Väisälä frequency (or buoyancy frequency) is a characteristic frequency of buoyancy waves in a stratified fluid, specifically the frequency of the oscillation caused by a vertical displacement of a parcel of fluid within a still body of fluid whose density varies with depth, lying stably within a gravitational field. The concept is used in meteorology and other sciences and applies to stars and planet atmospheres. It is defined as:
N² = -(g/ρ0) ∂ρ/∂z
(Often the equation is given in terms of a temperature rather than density: I presume this is because of the relationship between them if the ideal gas law holds.) This is the frequency of oscillations that form if there is a disturbance, and example being air pushed over a the top of mountain, and wavering up and down a bit even after it is passing over flat land again.