From AMS Glossary
The slight local difference between the gravitational attraction of two astronomical bodies and the centrifugal force that holds them apart.
These forces are exactly equal and opposite at the center of gravity of either of the bodies, but, since gravitational attraction is inversely proportional to the square of the distance, it varies from point to point on the surface of the bodies. Therefore, gravitational attraction predominates at the surface point nearest to the other body, while centrifugal "repulsion" predominates at the surface point farthest from the other body. Hence there are two regions where tide-producing forces are at a maximum, and normally there are two tides each lunar day and solar day.