One MP Cost is a recurring ability in the series. Though it was not named until Final Fantasy X, it has been around since Final Fantasy VI. The ability enables the user to use techniques and spells they have learned or are able to use at the mere cost of 1 MP, and is usually obtained extremely late in the game due to its potency.
It is never named in-game, but it is the ability of the Celestriad Relic.
One MP Cost is an ability that can be found very rarely on dropped weapons, and can also be customized by using 20 Three Stars. It will appear after fully powering up Yuna and Lulu's Celestial Weapon.
One MP Cost is only available on the Font of Power Garment Grid after going through all gates.