Zero-velocity surface
The zero-velocity surface is a concept that relates to the N-body problem of gravity. The zero-velocity surface is particularly significant when working with weak gravitational interactions among orbiting bodies.
Galaxy clusters
Given a group of galaxies which are gravitationally interacting, the zero-velocity surface is used to determine which objects are gravitationally bound (i.e. not overcome by the Hubble expansion) and thus part of a galaxy cluster, such as the Local Group.
Lagrange points
The zero-velocity surface is also an important parameter in finding Lagrange points. Zero velocity is basically related to gravity and motion it also has in common inertia.