The Interplanetary Medium (IPM) is matter such as Dust, gas, and Plasma (Solar Wind). within the Solar System between planets. The density in the vicinity of Earth is about 5 Particles per cubic centimeter. Given the electrically-conducting plasma, a Magnetic Field is maintained, forming a Dynamo.
Analogous matter between stars is called the Interstellar Medium and between galaxies, the Intergalactic Medium.