Defense vs. Missiles is a special modifier found on some Unique Items and Set Items in the Lord of Destruction, it adds a set amount to Defense against all ranged/projectile attacks, and stacks with the Amazon skill Avoid. It works against arrows, bolts, quills, javelins, and projectile elemental attacks. It does not work on damage over time spells like Fire Wall or Poison cloud.
In Diablo III, a similar concept is present in the form of reduced damage from ranged attacks, a secondary attribute that can occur randomly on several types of items.