Polarized hull plating
Polarized hull plating is a defensive technology used on Earth starships in the 22nd century. Supposedly an electromagnetic field of some sort reinforced the ship's hull, helping it resist damage. For all practical purposes, it served as a low-performance defensive shield, although it actually seemed to function more like a structural integrity field.
Because in the show it is treated the same as shields, we end up with the dubious situation of being able to bring downed sections of the armor "back online" by applying extra power to the system. For a better breakdown of why this is stupid watch Chuck's review of the Enterprise pilot.