Hit points (often abbreviated "HP") are a measure of how much damage a character, vehicle. or structure can withstand before becoming useless. It's a hold-over from the simulation wargames that are the parents of modern role-playing games. Historically, it was Dave Arneson (the one who isn't Gary) who got the idea from some American Civil War navy wargames, because having your hero-characters die on the first hit sucked ass.
What Does It Mean To Lose A Hit Point
Hit points are an abstract thing (some systems, like Dungeons and Dragons, allow a player to have negative hit points), which freaks people out when they're playing the game as a simulation. "Okay, so I lose 3 hit points. Does that mean my shirt's ripped? I dodged it? I got a cut on my arm? What?" It gets worse when characters get more hit points from getting more power, skills or experience, but that doesn't change their appearance. It gets ridiculous when a character gets so many hit points that highly improbable events become common (i.e.: a level 13 fighter in Advanced Dungeons & Dragons has a better-than-half chance of surviving a fall from terminal-velocity heights).
Here are some explanations people use to explain hit points, and their ablative nature. Most DMs use a mixture of a few of these:
- They're a measure of fatigue, as the combatants parry and dodge blows. Only the last few hit points represent actually getting hurt.
- They're a measure of the effectiveness of the equipment the character uses to protect themselves; armor straps are cut, weapons get nicked, metal plates get dented. "Rest periods" include repair and maintenance of their gear.
- In the case of giant fighting robots, or spaceships firing on each other with lasers, the hit points are a measure of the ablative armor evaporating to rid the excess heat before it causes damage.
- The heroes of the story are lucky, since they are protagonists, but their luck can run out.
- They measure actual injuries; the first few hit points lost are nicks and cuts, bruises and bumps, the last few are arrows lodged in your skull and your arm getting lopped off.
- Dungeons & Dragons 3rd Edition characters become disabled when they reach 0 HP, able to make only one action per turn. If they take a standard action while disabled, they automatically lose a hit point. If a character has the Die Hard feat, they may continue to take actions normally, otherwise they fall unconscious at -1, and begin dying. While dying, the character loses one hit point per round until stabilized. A character may stabilize on his own if the player rolls 1 on a d10, otherwise the character will need outside medical attention (a standard Heal check or any application of magical healing will automatically stabilize a dying character). Once stable, the same d10 roll determines whether the character regains consciousness.
- If a character's hit points are reduced to -10 for any reason, the character dies. If a character takes at least fifty points of damage from a single attack, he must make a DC 15 "massive damage" Fortitude save or die from the shock. The DMG has a chart for alternate massive damage thresholds based on creature size, the reason being that a blow that could kill a halfling in one shot isn't reasonably able to also kill an adult dragon.
- By RAW, drowning immediately sets HP to one. This interpretation is what makes the Omniscifier build work (take infinite damage to get an infinite bonus, then drown to get HP to zero and survive).
- Dungeons & Dragons 4th Edition has characters become unconscious and dying when they reach 0 HP, and must be stabilized before they die. Until an ally uses their turn to stabilize them, the dying character must make a Saving Throw (roll d20, hoping to get 11-20) each turn to stabilize themselves. A character who has dying status for three turns is dead, even if they aren't three turns in a row; taking an eight hour "extended" rest resets the count ("three strikes yer out"). Dying character will still take damage from attacks, environment or area effects, and will be immediately dead if they take half their maximum HP after reaching 0 hp, even if they still have dying saving throws left or they were stabilized (e.g. a character with 24 hit points is unconscious and dying at 0, and immediately dead at -12 hit points). A common cause of death in 4e is ongoing damage that continues after they fall to 0hp, or area attacks while dying.
- FATE System has two ways to allocate hits. The first is a line of check-boxes called "Stress counters" who are filled up when you receive damage and removed after the encounter. This measures how pressed the character is during combat, but it isn't anything but superficial damage to gear and skin, nothing major, which is also why it gets removed so easily. Where Stress will knock your character out in the end if all boxes are filled up, Consequences are the hits the character takes, but ignores to keep on fighting and to not take Stress. The problem with these are that they are Aspects - they are significantly harder to remove and can be invoked by your opponent to give them an edge on you. Examples could be "Bloody Nose" for a Low Consequence to "Oh, So That's My Kidney." for a Severe one. You can also take an Extreme Consequence that will negate a lofty 8 hit, but will also replace one of your Aspects with your new you-should-have-died-but-no-no-I-am-a-PC-I-can't-die Aspect, which could be considered comparable to burning fate points or Edge to survive certain death in other systems.
- Shadowrun has two separate tracks for damage, one for physical and one for stun. These tracks don't increase with experience, but are derived from the Body and Willpower attributes. Characters take penalties to all dice pools as damage accumulates, passing out when a track is filled. Stun overflow becomes physical, and a character can take a certain amount of physical overflow (again based on Body) before dying.
- Characters in Dark Heresy and its sister games have Wounds that remain static throughout their careers without taking Sound Constitution advances (gaining you one extra Wound in a system where you have around a dozen, though the advance can be taken multiple times). Once characters lose all their Wounds, any further damage is Critical damage, which refers to those wonderful Critical Hit tables we like so much.
- Characters in the New World of Darkness have a series of health boxes that is determined partially by their Stamina stat. Damage is divided into three types; Bashing, Lethal and Aggravated. This effects how long it takes to recover and just how serious the damage they have taken is; a character whose health boxes are filled with Bashing damage has been knocked unconscious, whilst one with them filled with Aggravated damage has been reduced to a pulp and is dead.
- Players in Star Wars D20 have two kinds of hit points, to better reflect the Star Wars setting and clarify just what the hell a hit point actually means. Vitality points are essentially plot armor points, representing both mooks' terrible aim and the energy to evade strikes from important characters. Force users also spend vitality when they use their powers. Wound points, on the other hand, represent actually getting seriously hit and are only removed by crits and atttacks suffered when you have no vitality points. Vitality points increase with your character level, but your maximum wound points are never greater than your Constitution score.
- In 7th Sea, PCs and other non-brute squad characters have both Flesh Wounds and Dramatic Wounds. Flesh Wounds don't actually cause any problems, but every time you take them you need to make a Wound Check via the Brawn stat, and if you roll less than your current Flesh Wounds you take a Dramatic Wound instead. This resets your Flesh Wounds to zero, but causes a stacking penalty, ending in you lying unconscious in a puddle of your own blood if you've got more Dramatic Wounds than your Resolve stat. Since 7th Sea is a game of swashbuckling action, this represents the difference between injuries that only make you look more badass and injuries that have actual dramatic weight to them.
See also: "What the fuck is a healing surge ?"