A health bar, a possible representation of the health of a character.
Health or vitality is an attribute assigned to entities such as characters or objects within role-playing games and video games, that indicates their continued ability to function. Health is usually measured in hit points or health points, shortened to HP which lowers by set amounts when the entity is attacked or injured. When the HP of a player character or non-player character reaches zero, that character is incapacitated and barred from taking further action. In some games, such as those with cooperative multiplayer and party based role playing games, it may be possible for an ally to revive a character who has reached 0 hit points and let them return to action. In single player games, running out of health usually equates to "dying" and (in the case of a player character) losing a life or receiving a Game Over.
Any entity within a game could have a health value, including the player character, non-player characters and objects. Indestructible entities have no diminishable health value. Health might be displayed as a numeric value, such as "50/100". Here, the first number indicates the current amount of HP an entity has and the second number indicates the entity's maximum HP. In video games, health can also be displayed graphically, such as with a bar that empties itself when an entity loses health (a health bar, typically red), icons that are "chipped away", or in more novel ways.
Dungeons & Dragons co-creator Dave Arneson described the origin of hit points in a 2002 interview. When Arneson was adapting the medieval wargameChainmail (1971) to a fantasy setting, a process that with Gary Gygax would lead to Dungeons & Dragons, he saw that the emphasis of the gameplay was moving from large armies to small groups of heroes and eventually to the identification of one player and one character that is essential to role-playing as it was originally conceived. Players became attached to their heroes and did not want them to die every time they lost a die roll. Players were thus given multiple hit points which were incrementally decreased as they took damage. Arneson took the concept, along with armor class, from a set of a naval American Civil War game's rules.
The U.S. Navy used a similar concept in their tactical war games already in 1920s and 1930s. In their simulation, each ship had a "life" parameter. The unit of Life of the ship was a number of "equivalent penetrative 14-inch shell hits". The Navy considered, e.g., that a Kongō-class battlecruiser had 12 Life points and a Nagato-class battleship had 18.8.