Armor

From EVE Wiki

The armor is the second line of defense for spaceships, after the shield. Once the shield has been destroyed, the armor plates covering the surface of the ship protect it. Unlike the shield, armor does not automatically regenerate itself. It must be repaired by using repair modules and drones. It can also be repaired for a cost in a station's repair shop.

Once the armor is destroyed, the ship's hull will start taking damage.