A mana bar or magic bar, used to keep track of a character's magic points (MP) in a video game.
Magic or mana is an attribute assigned to characters within a role-playing or video game that indicates their power to use special abilities or "spells". Magic is usually measured in magic points or mana points, shortened as MP. Different abilities will use up different amounts of MP. When the MP of a character reaches zero, the character won't be able to use special abilities until some of their MP is recovered.
Much like health, magic might be displayed as a numeric value, such as "50/100". Here, the first number indicates the current amount of MP a character has whereas the second number indicates the character's maximum MP. In video games, magic can also be displayed visually, such as with a gauge that empties itself as a character uses their abilities.
The magic system in pen-and-paper role-playing games such as Dungeons & Dragons is largely based on patterns established in the novels of author Jack Vance. In this system, the player character can only memorize a fixed number of spells from a list of spells. Once this spell is used once, the character forgets it and becomes unable to use it again.