Health is an effect that allows a player to take multiple lashings of damage from foes or the environment. It is of a limited quantity and can generally only be replenished through seeking first aid kits or by waiting a certain amount of time behind cover to recover. It is often visually conveyed to the player by either a bar that decreases in length as health is depleted, or by a number on the HUD representing the percentage remaining, or the number of hit points left. Some games such as the Call of Duty series choose not to show a distinct amount of health, instead displaying the well-being of the player through means augmented within the gameplay such as an audible heartbeat and loss of colour from the screen. Health is often supplemented by armour; a similar quantitative device to represent how much more of a beating the protagonist may take. Certain games combine regenerative health with non regenerative health, in that they allow the player to heal to a point, after which, they must use some form of item that recovers lost health. Some health systems include injury.
Note that there is a separate hit points concept for games that don't call it "health".
Double Life Bar/Hidden Life Bar
This entity is most often seen when encountering bosses within a video game. The player is chipping away at the boss life bar, getting it very near the empty mark. The player will deliver what they see to be as a death blow to the aforementioned boss character, only to have said boss character's life bar refill instantly, often times in a different color to signify the player has reached the second stage of the boss fight.