A trinary star system or stellar system is a system of three stars that orbit each other, bound by gravitational attraction.
In a triple star system, each star orbits the centre of mass of the system. Usually, two of the stars form a close binary system, and the third orbits this pair at a distance much larger than that of the binary orbit. This arrangement is called hierarchical. The reason for this is that if the inner and outer orbits are comparable in size, the system may become dynamically unstable, leading to a star being ejected from the system.