Gravity waves are waves in a fluid that obtain their restoring force from their buoyancy relative to the surrounding fluid. See [0] for the gory details.

Some examples of gravity waves in the atmosphere: [1] [2]

[0] http://glossary.ametsoc.org/wiki/Gravity_wave [1] http://cimss.ssec.wisc.edu/goes/blog/archives/2051 [2] https://www.youtube.com/watch?v=yXnkzeCU3bE

It's funny that you point this out, since some people in the field of astrophysics are quick to correct others when "gravity waves" are used when they really mean "gravitational waves", and sometimes vice versa.

Basically, gravity waves (or g-waves) are a type of perturbation in stratified media where the restoring force on sound waves is buoyancy. The other scenario is where the restoring force is pressure, which are pressure waves (or p-waves).

Gravity waves are important in not just planetary atmospheres but stellar media as well, particularly in the outer region of stars. A popular candidate for /gravitational/ wave sources is binary neutron star systems, so the two words aren't interchangeable there since they refer to two very different phenomena!

Are you mixing gravity waves and gravitational waves? Gravity waves are any waves in media in which gravity acts as restoring force, like waves in ocean, or waves between layers with different densities in atmosphere, or, AFAIU, whole atmosphere (think of atmosphere as a big shallow pool, with air sloshing around in the pool instead of some liquid).

https://en.wikipedia.org/wiki/Gravity_wave