A numeric representation of a character's magical energy.
MP is used for casting most types of magic. Some classes of magic have zero MP costs, such as Ninjutsu and Songs. For other categories, each spell requires a certain amount of MP to be used in order to cast it. The player will be unable to use the spell if current MP is insufficient. The maximum amount of MP available is determined by the player's race, job, level, merits, and certain equipment.
MP can be replenished by healing; the spells Refresh, Aspir, MP Drainkiss, or Magic Hammer; the songs Mage's Ballad or Mage's Ballad II; Evoker's Roll; an optional Sanction or Sigil bonus; certain equipment, medicine or food granting a Refresh Status Effect; or the weapon skills Starlight, Moonlight, Energy Steal, Energy Drain, Mystic Boon and Spirit Taker. Certain equipment with Refresh Effect as well. Stepping inside your Mog House or changing Jobs at a Nomad Moogle will also replenish your MP.
The Summoner job has two additional uses for MP. MP is used by Summoners for Avatar Perpetuation Cost; that is, Summoners use MP to maintain their avatars' presence during battle. When an avatar fights by a Summoner's side, the Summoner's MP is drained according to that avatar's perpetuation cost, after equipment effects and refresh effects are considered. Additionally, Summoner uses MP for Blood Pact: Rage and Blood Pact: Ward abilities, which are pet commands issued to avatars.
Some enemies, primarily those in the Arcana family, are sensitive to magical energy, and will attack nearby players that attempt to cast magics that consume MP (magic aggro). Ninjutsu and songs, however, do not cost any MP, and may be freely cast without causing such enemies to attack you.
A Black Mage may temporarily drop the MP costs of all spells to 0 via the use of his or her Two Hour Ability, Manafont. With Manafont active, they are permitted to cast spells for which they would have insufficient MP, and even if their MP is 0.