Armor Class (AC)
A creature's AC is a number that represents how well it avoids being wounded from a melee attack, ranged attack, or spell that requires an attack roll. The higher the AC, the better.
AC measures both toughness (an arrow strikes but fails to penetrate your armor) and agility (you dodge the swing of a sword).
Certain armor limits or prevents application of a Dex. modifier to AC. See the Armor Table.
Unarmored Defense class feature
You successfully hit your enemy if your attack roll, with all modifiers, equals or exceeds its AC.