= Gravitationally bound system
{title2=$\varepsilon=\tfrac12v^2-GM/r<0$}
A system whose mutual <gravity> confines its constituents rather than letting them separate indefinitely. In an isolated <two-body problem>, the relative <specific orbital energy> $\varepsilon=v^2/2-G(M_1+M_2)/r$ is negative exactly for bound, noncollisional <Kepler orbits>; $\varepsilon=0$ is the marginal escape case. External forces, collisions and time-dependent potentials require additional qualifications. The separation of a <gravitationally bound system> is determined by its equations of motion and need not follow the cosmological <Hubble flow>.
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