• (n.) That point, in the real or apparent orbit of one star revolving around another, at which the former is nearest to the latter.
The point in the orbital motion of a binary star system when the two stars are closest together.
The point of closest approach of two stars, as in a binary star orbit.
Per`i·as'tron noun [ New Latin , from Greek ... about + ... a star.] (Astron.) That point, in the real or apparent orbit of one star revolving around another, at which the former is nearest to the latter. 
periastron That point, in the real or apparent orbit of one star revolving around another, at which the former is nearest to the latter.
When two stars that orbit each other are at their closest point.