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A gravity wave in fluid dynamics is a wave such as an ocean wave, 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. Within Earth atmosphere, along with gravity, such waves are further influenced by the affects on temperature and density of the shifting altitude of the air. Gravity waves can also be found on the surface of and within the Sun and other stars (g-modes).
A similar-sounding term, gravitational wave is entirely different: it is a wave (oscillation) of the gravitational field itself rather than a wave-motion of material for which gravity is the restoring force.
Note that I have seen the term gravity wave used for gravitational wave in non-technical literature.