Ablative armor

Ablative armor was a type of protective hull plating used primarily on starships, and later on deep space stations (circa 2835), which possessed a capability for rapidly dissipating the energy impacts from directed energy weapon fire. Ablative armor which was developed by Starfleet in the late 24th centure, was designed to provide an additional layer of defense to starships.

History
Starfleet began use of ablative armor technology by 2371. It was first used on the USS Defiant, the prototype of her class, during the vessel's commissioning as a station defense complement of Deep Space 9 after first contact with the Dominion and the Jem'Hadar. Later, after satisfactory field tests with the Defiant, the technology was used on Prometheus class tactical assault cruisers.

Ablative Armor Plating
A key difference between simple ablative armor as used on the Defiant class and Prometheus class and the ablative armor plating used by the Intrepid class USS Voyager in 2377 was that the old armor was a simple alloy layer that eroded into a diffractive cloud as it was retarded by the phaser beam and then permanently diminished, while the generated ablative armor plating was a series of hull-mounted replicators and transporters that continually rebuilt a completely new hull of armor around the entire hull of the ship as the armor hull is damaged.

The ablative generator was designed to protect starships by generating a nearly impenetrable hull armor, which was deployed from the externally mounted generators, that when deployed spread across the ship's entire hull, following its contours. Though phasers could not be fired while the ablative armor plating was deployed, apertures allow torpedo launchers for the use of torpedoes. Though designed specifically to defend against the Borg, it was shown to be highly effective against other types of weapons fire. With armor deployed, Admiral Janeway's shuttlecraft was able to withstand disruptor fire from a 25th century version of a Klingon Negh'Var warship without any loss in armor integrity.