gas cloud
/ˈɡas ˌklaʊd/noun
1 Astronomy. A mass of gaseous material in space; specifically one from which a planet, solar system, or other celestial object is formed.
2A coherent mass of (especially harmful) gas or gaseous material released into the air, e.g. from a volcano, leaking gas pipe, etc.; especially a mass of poison gas released in a gas attack.
Origin
Late 19th century. From gas + cloud.