1. The effect arising from the differential gravitational effect of one body on another.

This usually manifests itself in the distortion of the shape of the body; especially, the surface layers.

2. Periodic changes in the shape of a planet, moon, or star caused by the gravitational attraction of a body near it.

The moon tugs on earth's oceans, causing high and low tides; while Jupiter's gravitational attraction on its moon Io causes ground tides; and when two stars are very close together, they pull each other's atmospheres into distorted shapes.