EXP share [ee-ex-pee-shehr]
1. Pokémon. a) This item, when held by a Pokémon, allows it to earn half the experience gained from defeating a Pokémon, regardless of if it was used in battle or not.
b) In Pokémon X & Y, the item is now a Key Item and can be turned on or off. Doing so results in all Pokémon receiving the experience. They receive 100% of the experience if in battle, and 50% of the experience if they were not in battle.
Heck, when they change any item in Pokémon, it's a little worrying but, and I gotta hand it to them, they usually change things for the best.