The main defence of Earth's NX-class starships in the 2150s, in which the hull plates are electromagnetically polarised to resist weapons fire, before deflector shields were available.
Polarised hull plating protects Enterprise NX-01 throughout Star Trek: Enterprise. Earth has no shields in the 2150s, so when danger threatens Captain Archer orders the plating polarised, which hardens the hull against energy weapons and impacts.
Its strength is reported as a percentage, and many battles turn on Malcolm Reed's warnings that the plating is failing or down. A hit on unpolarised plating can open the hull, as the ship learns in its early encounters with the Suliban and the Klingons.
The plating is outclassed by the shields of the Vulcans, Andorians and Klingons, and the heavy damage Enterprise suffers in the Xindi conflict, above all in Azati Prime, shows its limits. Shields had replaced it by the time of the Original Series.