This effect keyword was introduced in the Call of the Crystals set.
Defender is a keyword passive ability (written in bold in the effect area of the card).

• Defender is a specific keyword reserved for heroes and allies.
  
• Definition:
  
  • Defender allows the hero or the ally with the ability to do its combat damage first when being attacked.
    
  • Should the defender do enough damage to kill the attacker, the attacker will be killed before getting to do any damage.
    
  • An ambush hero or ally will bypass the defender's ability.
    

• Note that hero or ally who gained Defender with an attachment or an item will lose Defender if the card that gave the effect leaves the play.

(you may see the Attack rules for a more in depth explanation)

Cards related with Defender are: