Hit Points

Health Points (HP) are a standard in most games as a way of gauging how much damage a person can withstand. Once this parameter reaches zero, the character is either rendered incapacitated or dead, depending on the rules of the game.

In Sword Art Online
In the official version of Sword Art Online, health points are of dire importance, since 10 seconds after a player's HP bar reaches 0, the player's character would be destroyed and the player controlling the character would be killed by the NerveGear frying the player's brain. Health points in the game can be restored by using Healing Potions or Healing Crystals. HP in the game is displayed both in numbers (current HP/maximum HP) and a visual coloured bar, that changes from green to yellow (50% or below maximum HP) to red as the player's HP decreases. The maximum value of a player's HP value can be increased by leveling up.

In ALfheim Online
As ALfheim Online is a skill-based game instead of a level-based game like Sword Art Online, the HP system is maintained but a player's HP value isn't very high and can only be increased by a very small amount, most likely through skill upgrades. A player's health points can be restored in new ways, such as healing spells, with lower ones being common to all races and high level spells being allowed only to the Undine race.

In Project Alicization
In Project Alicization the classic HP bar was changed into a "Durability" parameter that the Underworld Residents refer to as «Life» (天命). Unlike in usual games, the Life of a person or object is not displayed in the user's interface and instead can be checked by calling out a system window called a Stacia Window. Furthermore, unlike usual games, Life can drop at an exponential rate through bleeding if it is not stopped in a timely manner by either using certain Sacred Arts rituals or using cloth to seal off the wound. Additionally, the user becomes weaker as their Life decreases and, since almost all residents of Underworld are Artificial Fluctlights, once a person's Life reaches 0, the system will begin deleting the deceased Fluctlight from the LightCube mainframe. This process can be halted with the use of revival Sacred Arts, but such a ritual can only be used if the Fluctlight's avatar wasn't destroyed in an explosion and even after the ritual is used, the already deleted information cannot be restored, meaning that a user revived through such a method will at the very least lose some of their memories, though the consequences might be even worse, depending on how much information had been deleted from the LightCube before the user was revived. This process is most likely not fatal to natural Fluctlights, as they are not stored within the LightCube.

Lost Life can be recovered through the use of certain Sacred Arts, the most effective being the highly risky Life transfers from one person to another. Such rituals are only effective if the injured person's wounds are sealed beforehand, as Life loss through bleeding makes Life recovery arts highly ineffective. The maximum Life increases automatically with age until the age of about 25, after which the maximum Life starts gradually decreasing, until the person dies due to old age. Only Administrator-level Sacred Arts have the ability to stop the natural degeneration of Life and the ability to increase the maximum Life value beyond its natural level.

Trivia

 * While the anime portrays the HP bar in Sword Art Online as being green when above 50%, in the novel the bar is said to be blue.

Puntos de vida HP