Pickpocket is a Pokémon ability introduced in the fifth generation. It allows the user to steal an item if they make contact with an opponent. It is exclusively a Hidden Ability.
When a Pokémon with Pickpocket gets attacked by a contact move, they will steal the attacker's item assuming the user is not already holding an item. This occurs after stealing moves or abilities like Magician allowing the user to steal back the item. However, items will trigger before being stolen. Pickpocket cannot steal specialty items that changes a Pokémon like the Griseous Orb from Giratina or Mega Stones. Sticky Hold prevents stealing as well.