Signature card

A Signature Card is one (or more) particular card(s) a duelist uses in almost all their Duels and is most associated with them. Signature cards tend to be the most likely to have Alternate artworks. In Yu-Gi-Oh! 5D's, signature card tend to be Synchro Monsters. Most signature monster cards are also called Ace Monsters.

Some video games have a similar concept known as a Key Card, in which the player selects a certain card as their "ace".