An HII Region (HII, pronounced as "H two region" or "H two") is a Hydrogen (H) cloud that is partially ionized. Typically its Temperatures reach 10000K. They often have a 656.3 nm hydrogen Emission Line (H-alpha (Ha) of the Balmer Series). They indicate recent Star Formation (SF), including blue stars, and are thought to be formed by the young stars. The Orion Nebula and the Horsehead Nebula are examples of HII regions.
Other kinds of hydrogen clouds are the HI Region (HI) and the Molecular Cloud. HII regions are thought to be the result of hot stars that formed within a molecular cloud, then ionized the molecular cloud's hydrogen. Photodissociation occurs at the border between molecular cloud and HII region, causing a Photodissociation Region.
"HII" is an astronomy term for ionized hydrogen, II meaning singly ionized. The common symbol (e.g., for hydrogen) outside astronomy is H+.
HII regions have at times been used to determine distances, though I believe this is now considered unreliable. The theory is of the existence of a HII Region Size--Galactic Luminosity Relation, specifically, a relation between the Luminosity (L) of a Spiral Galaxy and the size of its largest HII Regions.
Core Collapse Supernova
HI Region (HI)
Interstellar Medium (ISM)
Three Dimensional Model