Polarized hull plating

From Memory Alpha, the free Star Trek reference

Polarized hull plating was a defensive technology that preceded shields. In a battle situation, electromagnetic power could be applied to the hull materials, making them several orders of magnitude harder. This technology was used on starships of the Starfleet such as the NX class in the 22nd century. Unlike shields, polarization cannot be amplified to reinforce a specific direction. The amount of polarization on the ventral sections of the ship is the equal on the dorsal. (Star Trek: Enterprise)

During the months the Enterprise was in the Delphic Expanse, polarization technology back on Earth had been improved by 12%. (ENT: "Home")

Polarizing a hull on a 24th century starship can be used as a method of disrupting a tractor beam or masking a starship's approach. (DS9: "In Purgatory's Shadow"; VOY: "Equinox, Part II").

The fictional technology derives from late 20th century discoveries in the real world of ceramics which become harder when an electrical current passes through them.