What is the definition of interstellar space?

Interstellar space is defined as the vast expanse that exists between the stars and solar systems in a galaxy. It is the region that is not bound to any particular star system and serves as the medium through which light and cosmic rays travel between these celestial bodies. Therefore, stating that interstellar space is the area between solar systems accurately captures its essence.

The other options do not reflect this definition: the space occupied by our solar system pertains to the immediate area influenced by the Sun and its planets, while the area close to a star specifically refers to the region surrounding a star, and the orbit of a planet describes a distinct path that a planet takes around a star. These definitions do not encompass the broader context of interstellar space, which is uniquely defined by its position between separate solar systems rather than within any particular one.