Interstellar cloud is the generic name given to an accumulation of gas,
plasma, and
dust in
our and other
galaxies. Put differently, an interstellar cloud is a denser-than-average region of the
interstellar medium. Depending on the
density, size, and temperature of a given cloud, the
hydrogen in it can be neutral (
H I regions), ionized (
H II regions) (i.e. a plasma), or molecular (
molecular clouds). Neutral and
ionized clouds are sometimes also called diffuse clouds, while molecular clouds are sometimes also referred to as dense clouds.