Here's a breakdown:

* Binary System: A system of two celestial objects orbiting around a common center of mass. Examples include:

* Stars: Two stars orbiting each other.

* Star and a Planet: A star and its planet.

* Two Planets: Less common, but possible.

* Primary: The larger and more massive object in the binary system.

* Secondary: The smaller and less massive object.

Examples:

* Sun-Earth system: The Sun is the primary, and the Earth is the secondary.

* Sirius A and Sirius B: Sirius A is the primary, and Sirius B is the secondary.

Important Notes:

* This definition mainly applies to binary systems.

* In some contexts, "primary" can also refer to the dominant object in a multi-star system (more than two objects).

* The terms "primary" and "secondary" are used for convenience and don't always reflect the actual physical properties or importance of the objects.