The word interplanetary is associated with various space terms. It may mean a medium, an established theory, or even space, much like the term intergalactic. This term also means, between or across planets and is considered the lowered version of the word intergalactic.
The interplanetary theory is a specific and established theory in astronomy that means planets are made up of certain elements formed together, and that the space between planets exists. These are mainly made up of atomic particles or substances ejected by the sun through solar winds or other means, and as well as dust particles from asteroids or comets.
Interplanetary space is defined as the space between different celestial bodies or planets. In these, spaces, gases, rocks, and dust particles compose this part of a planetary system.
An interplanetary medium on the other hand, is the raw material of space. It is what fills the solar system of the different components is has, and also fills our own solar system with which bodies of a larger solar system move such as the planets, comets, and the stars. This kind of matter is composed of different materials like space dust, cosmic rays and plasma due to the emission of solar winds coming from the sun.