In the interest of sustaining this worthwhile feature in the NG, I
offer:

barycenter (n): A theoretical point 810 miles below the surface of
planet Earth. This point is the rotational center of the Earth and
Moon. The gravitational attraction between Earth and Moon affect the
oceans and their tides; centrifugal forces result from their
revolutions around the barycenter.

( above definition by Lenfestey/Lenfestey)

The barycenter is a definite consideration when examining the physics
of tides, particularly the standing wave on the side of Earth opposite
the position of the moon.