A sphere of influence (SOI) is an area or region over which an organization or state exerts some kind of indirect cultural, economic, military or political domination. Also, in some areas of habitation, shopping or retail outlets or indeed destination outlets, have a sphere of influence over towns of certain areas, for example the Central Business District (CBD).
A sphere of influence (SOI) in astrodynamics and astronomy is the spherical region around a celestial body where the primary gravitational influence on an orbiting object is that body. This is usually used to describe the areas in our solar system where planets dominate the orbits of surrounding objects (such as moons), despite the presence of the much more massive (but distant) Sun. In a more general sense, the patched conic approximation is only valid within the SOI.