tidal force

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A differential gravitational force acting along an extended body as a result of the varying distance from a gravitational source to the different parts of the body, such as the force of the moon on the earth's oceans closest to and farthest from the moon.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics any of various gravitational forces acting on a body caused by variation in the distance between it and a source of a gravitational field; such forces due to the Moon are responsible for tides