• (n.) That point in the orbit of a double star where the smaller star is farthest from its primary.
The point in any orbit around a star that is furthest from the star. It usually refers to a companion's orbit about the primary in a binary star system. The converse of periastron.
The point of greatest separation of two stars, as in a binary star orbit.
Ap·as'tron noun [ Greek ... from + ... star.] (Astron.) That point in the orbit of a double star where the smaller star is farthest from its primary. 
The point of greatest separation of two stars, such as in a binary star system.
When two stars that orbit each other are as far away from each other as they can get.
the point at which the stars of a binary system are farthest apart (opposed to periastron).