Fainting

In the Pokémon games, fainting is when a Pokémon reaches 0 HP, meaning they've taken as much or more damage than they have remaining hit points. A Pokémon which has fainted is unable to battle until it has been healed.

Besides fainting from damage in a battle, Pokémon can run out of HP because of poison while their Trainer is walking around in the field (prior to Generation IV). Pokémon can also faint instantly if their opponent uses Destiny Bond or Perish Song on them.

Fainting will cause a Pokémon to lose happiness, and they may grow to resent their trainer if they faint often and spend a lot of time unconscious. In Pokémon Yellow, Pikachu returns to its Poké Ball when it has fainted. Though a Pokémon which has fainted cannot battle, it can still use any moves with field effects, such as Dig or HM moves like Fly.

Normal items, such as potions or status ailment healing items will not work on a fainted Pokémon. Instead, they either have to be healed at a Pokémon Center or brought back with a Revive, Max Revive or Revival Powder.

In the anime, fainting is referred to simply as being unable to battle. These Pokémon are not always unconscious, but merely do not have the energy or ability to battle, or cannot continue battling for some other reason.