From Bulbapedia, the community-driven Pokémon encyclopedia.
Ace Spec cards (Japanese: ACE SPEC Ace Spec cards) are a classification of trading cards that debuted in Japan with the release of the Freeze Bolt and Cold Flare expansions and have yet to debut in the English sets.
An Ace Spec card is a very powerful Item card that has a special drawback. There can only be one Ace Spec card in a deck. There are currently 13 Ace Spec cards.
List of Ace Spec cards