An astronomical body's Hill radius is the radius of the surrounding spherical region (Hill sphere aka Roche sphere) within which smaller bodies would tend to orbit the body, despite the gravity of some third body they are orbiting together. Beyond the Hill radius, the small body may be drawn out of its orbit. For example, the Moon is within Earth's Hill radius and its orbit is stable over billions of years. If it weren't within this radius, it would not retain its stable orbit around Earth but would eventually orbit independently around the Sun. A body's Hill sphere necessarily lies between the L1 and L2 Lagrangian points of the body and its host (Earth and Sun in this example). The formula for the Hill radius for a body orbiting another is:
r ≈ a(1-e)(m/3M)1/3
(In relation to the Earth example above, this formula describes the Hill radius from Earth's center, formed by the Earth and Sun, affecting the orbits of objects orbiting Earth such as the Moon as well as artificial satellites such as the Hubble Space Telescope.)
The Hill radius is sometimes used as an approximation of how close to a smaller object might be to a body to be accreted, but the Bondi radius is another such calculation based on other factors.
The term tidal radius is often used when discussing gravitationally-bound groups of objects such as an entire stellar cluster or galaxy: the dynamics of a body within the group's tidal radius is determined by that group rather than some other entity.