HP

From FFTogether

HP, standing for Health Points is a stat known throughout the Final Fantasy series and other RPGs. Each character and monster you face has a set number of HP. During a certain point, the HP bar may turn yellow when you are HP critical. If the HP reaches 0, then your character is knocked out and unable to take any action until revived. HP can be healed with the Cure spell or Potions. Regen can restore HP over a certain amount of time. Certain equipment (mainly armors or accessories) raises the HP stat. In Final Fantasy X, HP can only be increased by equipping certain armors and raising HP through the Sphere Grid. In Final Fantasy XII, HP bonuses can be found on the License Board. In Final Fantasy XIII, similar to Final Fantasy X: HP can only be raised through the Crystarium Grid.

It is Game Over if all party members HP reaches 0.

Phoenix Downs

If your character is KO'd, you can revive them with a Phoenix Down. However, Phoenix Downs do not fully heal your character and they only restore a small amount of HP. Most games only allow Pheonix Downs to recover a minimal amount of HP, but in Final Fantasy X it can restore 50% of HP.

Reviving Spells

There are also spells that revive your allies known as Raise (also known as the Life Spell in early titles in Final Fantasy) which restores a little bit more than a Pheonix Down. The upgrade of Raise known as Arise, which fully heals all the HP of a KO'd ally.