primary

A use of the term primary within astronomy is to indicate a central body of a gravitationally-bound system that has a substantial majority of the system's total mass, e.g., sufficient mass that the barycenter is within the body itself. The Sun is the primary of the solar system and the Earth is the primary of the Earth/Moon system. For binary stars, in some cases, one binary companion has so much of the mass that the term primary would be used. The term is not used for a supermassive black hole (SMBH) at the center of a galaxy (e.g., the Milky Way), which does not have nearly such a high fraction of the galaxy's mass.

The term primary is generally used in discussions of orbits, such as those including stars and/or planets, etc. Another term, host star (or just host) is also used to refer to the star that hosts a particular planetary system.

The term primary also may be short for primary mirror.