Interstellar clou 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) (ie. 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.