An Etymological Dictionary of Astronomy and Astrophysics

An → ionization front of → H II regions
whose expansion speed is comparable to the → sound speed in the gas (~ 10 km/sec for hydrogen at 10⁴ K). A D-type ionization front results from → R-type ionization front when its propagation speed decreases as the volume of gas ahead of the ionization front grows. If front velocity is equal to a lower limit (C₁² / 2C₂, where C₁ and C₂ are the sound speed ahead and behind the front respectively), the front is called D critical.

See also: D referring to a dense gas; → type; → ionization; → front.