Unable to battle
Being unable to battle (Japanese: 戦闘不能 sentō funō) is a condition that occurs when a Pokémon receives enough damage so that it can no longer participate in a battle. In the anime, a Pokémon is shown to be unable to battle when its eyes are "swirly" - presumably to emulate dizziness. During a Pokémon League match, once a Pokémon is deemed unable to battle by an official League judge, it must be recalled and cannot be used again; however, this only true in the Pokémon anime and some of the manga, not the games.
In the English version of the games, the term is translated, a little more loosely, as the faint status. This occurs when a Pokémon's Hit Points are depleted.
This can happen in any number of ways, though the most common is to merely attack the opponent with moves like Tackle, which inflict damage without other side effects.
However, attack power isn't the only way to faint a Pokémon, as there are a few other status conditions which can do the trick. Confusion is a simple way, as it will sometimes cause the confused Pokémon to hurt itself when attacking. Another good way is to cause the Pokémon to receive a Burn, which will both lower its attack power slightly, as well as cause damage every turn to it. The most common way, though, is to Poison a Pokémon, which not only lowers their HP in battle like a Burn, but also causes one HP of damage for every step the trainer takes outside of battle.
