A planetary class covering small, barren, airless bodies such as planetoids and moons, most famously the lifeless Regula, inside which the second stage of the Genesis experiment was carried out.
Class D is the classification for small rocky bodies, planetoids and moons with little or no atmosphere and no native life. Such worlds are of interest chiefly for mining, for outposts or as test sites, since nothing lives there to be harmed.
The best-known example is Regula in Star Trek II: The Wrath of Khan. The planetoid is described as Class D, a great lump of rock and ore, which is why Carol Marcus's team chose its interior for the second phase of Project Genesis, creating a cavern full of life inside a dead world.
Enterprise adds another case in Cease Fire: Weytahn, called Paan Mokar by the Vulcans, was an uninhabitable Class D planetoid until Andorian settlers terraformed it, after which both powers claimed it and Jonathan Archer was asked to mediate.