H I region
INTERSTELLAR CLOUD COMPOSED OF NEUTRAL ATOMIC HYDROGEN
H. I. Region; HI region; H I profile; H I; HI Region; Hydrogen cloud
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.