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. Such dust is known as interplanetary dust particles (IDP).
Analogous matter between stars is called the interstellar medium and between galaxies, the intergalactic medium.