Red Health, also known as Provisional Damage, is a term used in fighting games to indicate recoverable health.
In games where Red Health exists, whenever a character takes damage from an attack, a section of their health bar indicating the damage received will be highlighted (usually in red). This shows the amount of health which can be recovered, with the method of doing so differing depending on the game. Methods of recovering this Red Health include tagging out in the Marvel vs Capcom games, or by activating Heat or Blood Heat in Melty Blood. Some games, such as Street Fighter IV and Street Fighter X Tekken allow Red Health to be slowly restored as the match progresses, even if the character doesn't tag out. If a character is hit by another attack before the entirety of red health can be recovered, the remaining Red Health that has not yet been recovered is lost.
In M.U.G.E.N, Red Health can be implemented into characters, but will not be visually displayed due to engine limitations. This can be circumvented with the use of custom life bars or manually implemented gauges, however.