Hit die

In the singular form, a hit die is the die rolled to generate hit points. In the plural form, hit dice (HD) is a measure of relative power that is synonymous with character level for the sake of spells, magic items, and magical effects that affect a certain number of hit dice of creatures.

Class hit die

HD	Class
d4	Sorcerer, Wizard, Arcane trickster, Arcane Scholar of Candlekeep, Red Wizard of Thay
d6	Bard, Rogue, Warlock, Assassin, Harper agent, Eldritch knight, Pale master, Shadow Thief of Amn, Invisible Blade
d8	Cleric, Druid, Monk, Ranger, Favored soul, Spirit shaman, Arcane archer, Neverwinter Nine, Shadowdancer, Stormlord, Sacred fist
d10	Fighter, Paladin, Blackguard, Divine champion, Duelist, Warpriest, Weapon master
d12	Barbarian, Dwarven defender, Frenzied berserker, Red dragon disciple

A character rolls one hit die each time he or she gains a new level, then applies any constitution modifier to the roll, and adds the result to his or her hit point total. Thus, a character has the same number of hit dice as levels. For his or her first hit die, a 1st-level character gets the maximum hit points rather than rolling (although constitution modifiers, positive or negative, still apply).

Example

Vadania gets a d8 hit die because she’s a druid. At 1st level, she gets 8 hit points instead of rolling. Since she has a constitution score of 13, she applies a +1 bonus, raising her hit points to 9. When she reaches 2nd level (and every level thereafter), Vadania’s player rolls a d8, adds 1 (for her constitution bonus), and then adds the total to Vadania’s hit points.

Constitution penalty

If your character has a constitution penalty and gets a result of 0 or lower after the penalty is applied to the hit die roll, ignore the roll and add 1 to your character’s hit point total anyway. It is not possible to lose hit points (or not receive any) when gaining a level, even for a character with a very low constitution score.