The maximum volume of star within which material is gravitationally bound to the star, when the star is part of a binary system. If the distance between the two stars is sufficiently small, the Roche lobe can become smaller than the actual volume of one of the stars, causing its matter to be gravitationally captured by the other star via an accretion disk. If the other star is a neutron star or black hole, such a system is called an X-ray binary. See also Accretion disk, Binary star, X-ray binary.

in binary system, the region of space gravitationally "owned" by one member of the pair (a planet inside the Roche lobe would orbit that star, not the system centre of mass).

An imaginary surface around a star. Each star in a binary system can be pictured as being surrounded by a tear-shaped zone of gravitational influence, the Roche lobe. Any material within the Roche lobe of a star can be considered to be part of that star. During evolution, one member of the binary star can expand so that it overflows its own Roche lobe, and begins to transfer matter onto the other star.