An interstellar cloud is a denser-than-average part of the interstellar medium, (ISM). It is matter and radiation in the space between the star systems in a galaxy.

An interstellar cloud is formed by the gas and dust particles from a red giant in its later life.

The cloud is made of gas, plasma, and dust in our and other galaxies.

Its hydrogen can be:

• neutral, making it an H I region;
• ionized, or plasma making it an H II region; or
• molecular (molecular clouds).

Neutral and ionized clouds are sometimes also called diffuse clouds.