Defensive Duellist is a Reaction obtainable by a Feat of the same name. If you are attacked while wielding a Finesse Weapon weapon, you can temporarily increases your 
Armour Class.
Description
When you are attacked with a melee attack while wielding a Finesse Weapon you are Proficient in, you can use a Reaction to increase your Armour Class by your Proficiency Bonus, possibly causing the attack to miss.
How to learn
Granted by feats: Defensive Duellist (Feat)