The interstellar medium (or ISM) is the name astronomers give to the tenuous gas and dust that pervade interstellar space.
Whilst the ISM refers to the matter (interstellar matter, also abbreviated by ISM) that exists between the stars within a galaxy, the energy, in the form of electromagnetic radiation, that occupies the same volume is called the interstellar radiation field (or ISRF).
The ISM consists of an extremely dilute (by terrestrial standards) plasma, gas and dust, consisting of a mixture of ions, atoms, molecules, larger dust grains, electromagnetic radiation, cosmic rays, and magnetic fields.
The matter consists of about 99% gas and 1% dust by mass.
It fills interstellar space.
This mixture is usually extremely tenuous, with typical gas densities ranging from a few hundred to a few hundred million particles per cubic meter.
As a result of primordial nucleosynthesis, the gas is roughly 90% hydrogen and 10% helium by number, with additional elements ("metals" in astronomical parlance) present in trace amounts.
The ISM plays a crucial role in astrophysics precisely because of its intermediate role between stellar and galactic scales.
Stars form within the densest regions of the ISM, molecular clouds, and replenish the ISM with matter and energy through planetary nebulae, stellar winds, and supernovae.
In turn, this interplay between stars and the ISM helps determine the rate at which a galaxy depletes its gaseous content, and therefore its lifespan of active star formation.