Original from: DEF
DEF refers to the Defense Points of a Monster Card. If a Monster attacks another with an ATK that is greater than the DEF of that Monster, the Defense Position Monster is destroyed, but the owner of the Defense Position Monster takes no damage to his/her Life Points, unless the attacking Monster has an ability (called "Piercing") where the difference between ATK and DEF is dealt as Battle Damage.
If the same situation is reversed, where the ATK is lower than the opponent's monster's DEF, both monsters remain on the field, but the owner of the ATK monster loses Life Points equal to the difference between the ATK and DEF values.
If both the ATK and DEF values are the same, neither monster is destroyed, and neither player loses Life Points.