HD — Hit Dice
The dice used to work out how much health a character or creature has, written like 2d8 + 2.
Hit Dice (HD) describe health as dice. A creature with 2d8 + 2 has the health of two eight-sided dice plus two. Characters usually have one Hit Die per level, set by their class. Some rulesets also let you spend Hit Dice to heal while resting.
In Fantasy RPG
Each Fantasy RPG class has a hit die from d6 to d12 that sets its hit points. A short rest heals one average hit die plus the Constitution modifier. Monsters list their hit dice next to their hit points, e.g. 7 (2d6).