also: エッチツーりょういき
noun
H II region
Astronomy term for a cloud of ionized hydrogen, typically found around young, hot stars.
オリオン大星雲は有名なＨＩＩ領域の一つです。
The Orion Nebula is one of the famous H II regions.
H I region: a cloud of neutral atomic hydrogen, in contrast to the ionized hydrogen in an H II region.
From the astronomical notation H II, where 'H' is the chemical symbol for hydrogen and 'II' indicates singly ionized hydrogen (H+). The Japanese term is a direct borrowing of the English term.