What exactly is a Hit Dice (HD)?

[Invisible]

In many abilities, spells the Hit Dice plays a role (e.g. Hypnotism). Sadly in the (very uncomplete) encyclopdia is nothing written about it.

[Mork]

Hit Dice (HD)
Hit Dice represent a creature’s general level of power and skill. As a creature gains levels, it gains additional Hit Dice. Monsters, on the other hand, gain racial Hit Dice, which represent the monster’s general prowess and ability. Hit Dice are represented by the number the creature possesses followed by a type of die, such as “3d8.” This value is used to determine a creature’s total hit points. In this example, the creature has 3 Hit Dice. When rolling for this creature’s hit points, you would roll a d8 three times and add the results together, along with other modifiers.

[Icedfate]

Hit Dice is the total number of dice that were rolled to determine your hit points.
as a player character on tabletop, each time you gained a level, you rolled a die to see how many hp you gained. that die you rolled is your "Hit Die".

so at level 1 rogue, you rolled a d6 and were comsidered to have "1 Hit die"
at level 2, you rolled another d6 and add it to the total. now you have "2 HD"

generally HD is = character level.
so , if you are level 5 monk/ level 3 vivisectionist, you have a total of 8 Hit Dice because you rolled the die 8 times, once per level.

this goes back to 2E, the Monster Manual used the term "HD", hit dice, because there was no such thinng as "monster level" or "challenge rating", the monsters had HD. like an enemy would say under their hit points "6 HD" which meant the Dungeon Master would roll 6d8 to see how many hp the monster had. monsters always roll a D8 for hp, unless it is specified otherwise.

weaker enemies, like goblins might have had 1/2 HD, meaning you divide the roll by 2.

some enemies might have had "6+6" under HD, which meant, roll 6d8 and then add 6hp to that results.
a 6 HD monster was considered "level 6" for purpose of spell immunity, like a cloudkill would instantly anything or anyone with 6HD or less. so level 6 player character would die instantly to a cloudkill, which is normally only casted by level 9 mages , so yeah, if you're level 6 and see a level 9 mage, you should be worried

edit : i got details slight wrong, cloudkill auto slays 3HD or less, 4-6 HD gets a saving throw, more than 6 HD you're immune to the death effect but still take CON damage.
i just realized now, that's what that was surrounding the Scythe Tree enemy on that one map i just cleared. Delay Poison, Communal FTW

[Invisible]

Thank you both for your explanation!

In case of "Hypnotism" the text is: "Roll 2d4 to see how many total HD of creatures you affect". This is a bit more complicated. If I understand this now right, it means the following: I roll a 2 and 3 = 5 HD so I can affect 1 creature with HD 5 aka Level 5 or 5 creature with HD 1 aka level 1. Maximum (with good luck) would be one HD 8 creature.

Is this correct?

[Zzyl_tsw]

Yes.
Now considering the "stats" of the creature ingame (you can check them with the inspect option), it means it will be useless VERY soon.
Also, if you see an ennemy with mutiple classes, you add them.
So level 5 Barbarian + level 3 Rogue means 8 HD.