DEF

From WikiFic

DEF is a shortened form of "Defense," the value that determines a Monster Card's resistance to attack.

If a monster in Attack Position attacks a monster in Defense Position and the attacking monster's ATK is the same as the Defense Position monster's DEF, neither monster is destroyed, and neither player loses Life Points. However, if the ATK value is higher than a Defense Position monster's DEF value, the Defense Position monster is destroyed, but no Battle Damage is dealt, unless the attacking player has a card with an effect that deals the difference between the ATK and DEF values as Battle Damage. Further, if the ATK value of an attacking monster is lower than the DEF value of the Defense Position monster, the difference between the ATK and DEF values are dealt as Battle Damage to the attacking player.

Without any monsters to defend against an attack, damage is dealt to the opponent's Life Points equal to the total ATK value (Also called a Direct Attack, which deals Direct Damage), if the attack is not blocked or disrupted by a Spell or Trap Card.