Gravity well

In physics, a gravity well is a distortion in space-time caused by a massive body such as a planet. The term is a reference to the 3-dimensional analogy of this phenomenon: an extrusion of an otherwise 2-dimensional sheet. An actual gravity well involves higher-dimensional bending.

The "depth" of a gravity well corresponds to the Δv required to leave -- also known as the escape velocity. Deeper wells require more Δv, and so it is harder for a rocket to escape from them (or to stop at the bottom). Deeper wells also tend to make for more efficient gravitational slingshots.