1. A point at which the stars of a binary system are farthest apart, as opposed to periastron, or "when a star is closest to the star it is orbiting".
2. The point at which an object, such as a planet or comet, is farthest from the star it is orbiting.

The orbits of planets moving around the sun, of satellites moving around planets, and of stars moving around each other are never circular, but they are always elliptical or rounded like an egg.

Because of such oval movements, there will be one point in the orbits of all of the astronomical bodies when they are farther apart than at any other time in their orbits; so, in the situations of two stars (binary system), this point of greatest distance is called apastron from two Greek elements: apo-, "far" and astron, "star".