A Power Bar (sometimes called a Super Combo Gauge or Super Meter) is a gameplay mechanic first introduced in Super Street Fighter II Turbo, and has been replicated in virtually every fighting game since.
In Super Street Fighter II Turbo, the Super Gauge is a bar of uniform length that exist for each character. The gauge begins at 0, and is filled by landing attacks, being hit by attacks, and executing special moves. Once the gauge is full, it will begin to flash as the word "SUPER" appears, indicating that a Super move may be used. Once the Super is used, or if the round ends without usage of it (but only in Super Street Fighter 2 Turbo) the Super Gauge resets back to 0. In later games, the Super Gauge would also be used in the execution of EX special moves, which are improved versions of special moves, but cost less meter than the usual super move.
In many fighting games, including M.U.G.E.N, multiple levels of Super Gauge exist, allowing the player to store meter in their power bar for more than one Super move. In M.U.G.E.N, the maximum amount of meter that may be stored in a character's power bar is defined in the character's cns file. In most cases, however, the value is usually set to 3000 power. Although there is no set value for how much meter corresponds to a single level of Super, it has become commonly accepted practice to treat 1000 power as 1 level of super meter.