HD redirects here. For the long-gone player race, see Hill Dwarf
Hit dice (or HD) is roughly equivalent to a monster's level. It is not the same as hit points. Hit dice affects monster's magic resistance, spellpower, experience value, etc. Is essentially the monster's experience level. Affects what a monster polymorphs into, and whether it will be affected by things like mephitic cloud and confusing touch.
Monster melee to hit is 18+HD*(1.5 or 2.5). The former for non-fighters, the latter for fighters. Beam to hit is 12*HD.
As some examples: hobgoblins and other D:1 monsters have 1 HD, ogres have 5, ancient liches have 27, Cerebov only has 21, despite his fearsome spells. The highest-HD monsters are orbs of fire at 30: that is why they are so damaging with their fire conjurations.