Hit dice

Hit dice (or HD) is roughly equivalent to a monster's level and gives the approximate hit points the monster has. Normal calculation for monster's HP is total of hit dice amount of 8-sided dice. Exceptions are golems, riders, special "fixed" HP monsters (which are usually uniques), adult dragons, elementals on their home planes, and HD:0 monsters.

When the player is polymorphed into a monster, the status line shows HD instead of hit points. Player's hit points are also usually HD times d8. Exceptions to this are golems, grey dragons, elementals on their home planes, and HD:0 monsters.