Polarized hull plating was a defensive armored hull technology used on shuttles and starships. Before the development of deflector shielding, hull polarization was the primary defensive measure for starships against enemy weaponry.
In the January 2002 -issue of Star Trek Monthly, according to André Bormanis: "To minimize potential damage from weapons fire and other space hazards, NX-01 incorporates a polarization matrix in its armored hull plating. Through the application of electromagnetic power, the metal hull of the ship can be made several orders of magnitude harder than it is in its non-polarized state."
Polarized hull plating was presumably invented by 2102, as that was the year the ECS Horizon was commissioned.
In the 24th century, starships still occasionally polarized their hull plating, though not typically as a defensive countermeasure. One example was disrupting a tractor beam lock 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 enemy sensor scans. Polarizing the hull was also done by downed vessels to help their mothership locate them. (DS9: "In Purgatory's Shadow"; VOY: "Once Upon a Time", "Equinox, Part II")