A HI region (read as "H one region") is a cloud of generally neutral atomic hydrogen, e.g., with only about 1 in 10,000 atoms ionized. Their typical temperature is 100 K, in which case the material is referred to as a cold neutral medium (CNM). Above that, they are unstable, but are stable again above 10,000 K. Between those temperatures, they generally cool or heat to a stable state. They typically also have some helium and other elements. They are not very luminous, mostly the 21-cm line, which along with the spectral lines of other elements embedded in the cloud, allows them to be detected.
Other kinds of hydrogen clouds include HII regions and molecular clouds. HI regions and HII regions are termed diffuse clouds and molecular clouds are termed dense clouds.