Of all the game elements introduced or popularised by Dungeons & Dragons the concept of hit points has probably been the most influential and widespread, finding its way into numerous tabletop and electronic games alike. The origin of the mechanism likely lies in the Chain Mail Fantasy Supplement where the superior fighting capability of heroes, wizards and monsters is represented by making them individually equivalent to multiple figures. For instance, a hero is worth four figures of any type, a wizard is equal to two armoured foot, or if mounted two medium horse, whilst a giant attacks as twelve heavy foot and defends as twelve armoured foot (or twelve heavy foot according to Fantasy Reference Table on p. 43). In each case, these powerful combatants are normally only slain after suffering enough cumulative or simultaneous hits to kill the number of men to which they are judged to be equivalent. That these were the forerunners of hit dice can be seen most clearly in the goblin, orc and hobgoblin entries, where they are indicated to attack and defend as heavy foot/light foot, heavy foot/heavy foot, and armoured foot/heavy foot, respectively, which is a relationship later reflected in their hit die ratings of 1−1, 1 and 1+1.
At an indeterminate but early juncture hits as kills were deemed insufficiently granular for swords & sorcery adventure gaming. Instead, each man equivalent was assigned 1-6 "hit points", a successful hit inflicting 1-6 damage rather than slaying outright. This approach had the advantage of allowing the average result of an isolated hit to remain a kill, but also ensured that five-in-twelve such hits would be non-lethal. It also created the possibility that combatants with multiple hit dice might be laid low with a single blow, if they had been unlucky in their hit point determination. Because of the way hits accumulate, the introduction of hit points strengthened the less powerful creatures and weakened the greater ones. Dealing with non-fatal hits is an area where Dungeons & Dragons often comes in for criticism, as its default assumption is that hit point loss in and of itself has no further deleterious effects. Since damage is most often conceived of as the inflicting of wounds, this seems counterintuitive, but it is worth recalling that the original edition of the game did give the subject some consideration, noting that "whether sustaining accumulative hits will otherwise affect a character is left to the discretion of the referee" (M&M, p. 18).
Both the first and second editions of Advanced Dungeons & Dragons take the time to address the issue of hit points and wounds, warning that debilitating injuries are "not the stuff of heroic fantasy" (1e DMG, p. 61) and that "characters have enough of a challenge as it is" (2e DMG, p. 74). Even so, it is nevertheless noted that this is not necessarily the case for monsters and in fact neither edition is completely opposed to the idea of inflicting specific wounds on characters. For instance, maiming is considered a viable alternative to death in cases where player characters have played well but been extraordinarily unlucky (1e DMG, p. 110), and the sword of sharpness is well known for its ability to sever limbs regardless of whether the hit points of the target have been exhausted. The hydra is a good example of a monster that suffers an injury for each hit die of damage suffered, in this case the loss of one of its heads. A more general example is extant for winged flying creatures, as it is specified that if such monsters lose more than half of their hit points they must seek to land (2e DMG, p. 78), whilst if they lose more than three-quarters of their hit points during flight they plummet to the ground (1e DMG, p. 53).
Lest we forget, the original Dungeons & Dragons game has rules for aerial combat, apparently borrowing from Fight in the Skies by Mike Carr, which involve specific body location and critical hits. Nor should the much maligned hit location system presented in Supplement II: Blackmoor, and its assignation of hit points to various body parts, be overlooked. By the same token, the first edition of Advanced Dungeons & Dragons includes a rule for handling attacks against combatants without helmets (1e DMG, p. 28), whilst the second edition discusses the inclusion of called shots (2e DMG, p. 58). Indeed, the methods by which hit points are commonly restored, such as healing magic, regeneration, or lengthy periods of rest, suggest that their loss is representative of wounds suffered, rather than luck, skill, endurance or divine protection. Whilst it might be reasonable to evade this conclusion by applying the retroactive logic that a character is not wounded unless healed, for which precedent exists with regard to saving throws against poison, such arguments are unlikely to satisfy anybody desirous of a cause and effect relationship. As with other elements in the Dungeons & Dragons combat system, hit points oscillate between having abstract and specific qualities.
In reality, any wound significant enough to impair fighting ability is likely to take an individual out of the combat they were involved in. On the other hand, the notable individuals who fight on despite injury are the very sorts that player characters are intended to emulate. Leaving things up to individual game masters as the original game does is the most coherent solution, but also runs the risk of seeming too arbitrary. The suggestion in the first edition Dungeon Master’s Guide that characters reduced to zero hit points suffer some sort of injury rather than being slain is attractive, especially if the negative ten optional rule is discarded or modified. A house rule used in the World of Silver Blade is that characters brought to zero hit points or below are wounded and out of the fight, suffering ongoing penalties until the injury is healed, regardless of hit point recovery. Furthermore, and partly because magic is less prevalent in the campaign, characters can heal one hit point for every turn of rest after combat up to a maximum of one point for every die of damage suffered. So, for example, a character fortunate enough to have survived a fire ball spell that inflicted 6d6 damage can expect to recover six hit points after resting and tending his wounds for an hour.
Clearly hit points are a useful abstract combat mechanism for swords & sorcery adventure games, as well as being a source of controversy that defies singular definition. As Gygax notes in the first edition Dungeon Master’s Guide it is ludicrous to suppose that characters can regularly survive multiple sword blows, but the binary "alive or dead" model that hit points seem to support seems equally unsatisfactory, and is a level of abstraction often gainsaid elsewhere in the text. Whilst it may be undesirable in a game of "heroic fantasy" for persistent or debilitating injuries to feature overmuch, a world without wounds is no more appealing. The key to reconciling this likely lies in realising that, although hit point loss may indicate injury and vice versa, the two are not inexorably related, which is to say a broken arm need not correspond to any form of hit point loss at all, and yet could be healed by restorative magic. Nonetheless, it is worth noting that healing magic in Dungeons & Dragons is bound up with the positive and negative energy planes, as well as the concept of life energy levels. Indeed, hit points are perhaps most usefully defined as "life force", but probably they are best understood as being whatever they need to be in the context of game events.