Polarized hull plating was a defensive armored hull technology used on starships. Before the development of deflector shielding, hull polarization was the primary defensive measure for starships against enemy weaponry.
In the 22nd century, this technology was used on Earth starships such as the J-class and the NX-class and on smaller auxiliary vessels such as the shuttlepods. During the months the Enterprise was in the Delphic Expanse, polarization technology on Earth had been improved by 12%. (Star Trek: Enterprise; ENT: "Horizon", "Vanishing Point", "Home")
In the 24th century, starships still occasionally polarized their hull plating, though not typically as a defensive countermeasure. One example was disrupting an enemy's tractor beam by rotating the frequency of the hull plating's electromagnetic field. Ships also sometimes polarized their hull plating to help mask their sensory signature, which worked by interfering with the enemy's sensors. (DS9: "In Purgatory's Shadow"; VOY: "Equinox, Part II")