Neutral atomic hydrogen in astrophysics is indicated by the abbreviation HI or H I (pronounced "H one"). The abbreviation HI is used in some indications of neutral hydrogen spectral lines such as the 21cm line, e.g., [HI].
It consists of a single hydrogen atom (i.e., not half of a hydrogen molecule) including a single orbiting electron. In space, with sufficient density, and sufficiently low temperature, (generally less than 50 K) it can form molecules. Regions of neutral hydrogen are generally below 100 K but near early stars, may be thousands.
The 21cm line is used to detect neutral atomic hydrogen at a distance, to detect HI regions and galaxies that include them.