The interstellar medium (or ISM) is a term used in astronomy to describe the rarefied gas that exists between the stars (or their immediate "circumstellar" environment) within a galaxy. This gas is usually extremely tenuous, with typical densities ranging from a few tens to a few hundredths of a particle per cubic centimeter. Generally the gas is roughly 90% hydrogen and 10% helium, with additional elements ("metals", in astronomical parlance) present in trace amounts.
The interstellar medium is usually divided into three phases, depending on the temperature of the gas: hot (millions of kelvin), warm (thousands of kelvin), and cold (tens of kelvin). This "three-phase" model of the ISM was initially developed by McKee and Ostriker in a 1977 paper, which has formed the basis for further study over the past quarter-century. The relative proportions of the phases is still a matter of considerable contention in scientific circles.
Features prominent in the study of the interstellar medium include molecular clouds, interstellar clouds, supernova remnants, planetary nebulae, and similar diffuse structures.
See also: Timeline of knowledge about the interstellar and intergalactic medium, Outer space.