H I region

Interstellar cloud composed of neutral atomic hydrogen

An HI region or H I region (read H one) is a cloud in the interstellar medium composed of neutral atomic hydrogen (HI), in addition to the local abundance of helium and other elements. (H is the chemical symbol for hydrogen, and "I" is the Roman numeral. It is customary in astronomy to use the Roman numeral I for neutral atoms, II for singly-ionized—HII is H+ in other sciences—and III for doubly-ionized, e.g.

From Wikipedia, under CC BY-SA. More on occurri.