A Gravity Wave in fluid dynamics is a wave such as a wave on water, specifically a wave on the surface between two fluids where the restoring force is Gravity. The surface can be less clear-cut than water/air such as an interface between two different densities of liquid or gas. Such gravity waves can be found on the surface of and within the Sun and other stars.
A similar-sounding term, Gravitational Wave is entirely different: a wave in a gravitational field rather than a wave involving gravity as its restoring force.
Gravitational Wave (GW)
Internal Gravity Wave
Rossby Radius of Deformation