A "gravity cloud" refers to a type of cloud formation resulting from atmospheric gravity waves. These clouds, sometimes called undulatus or billow clouds, are a product of atmospheric gravity waves and are the product of air being forced upwards. When air is forced upward, it cools. If there is enough moisture, water condenses and creates clouds.
In essence, the term "gravity cloud" isn't a standard meteorological term, but it aptly describes clouds formed due to the influence of gravity waves in the atmosphere.
