gravitationally bound

Two or more objects being "gravitationally bound" means they are close enough that gravity is keeping them together (attached or orbiting each other). The term describes two objects co-orbiting (e.g., the Earth and the Moon) or a whole system of objects, such as a planetary system (e.g., the solar system), a stellar cluster, a galaxy, a galaxy cluster, and some would reserve the term supercluster for a gravitationally-bound group of galaxy clusters. In addition to such systems/groups of objects, the gravitationally-bound object could be a gas cloud such as an accretion disk. The term also describes Earth with ourselves, as well as with anything else resting on Earth's surface. Separating gravitationally-bound objects generally requires energy to accelerate them to the escape velocity, i.e., giving give them kinetic energy that exceeds their gravitational binding energy. For more than two objects gravitationally bound, it is possible that during an interaction of three or more, some energy is swapped giving one enough kinetic energy to escape, which can be called being kicked out: for example, a comet might be sent out of the solar system or a star sent out of a globular cluster.