Ablative armor is a type of protective hull plating used on starships, which possesses a capability for rapidly dissipating the energy impacts from directed energy weapon fire. Starfleet began use of ablative armor technology by 2371. (DS9: "Past Tense, Part I")
Sometime in 2371, after her deployment to Deep Space 9 and unknown to Starfleet Operations, ablative armor was added to the USS Defiant. (DS9: "Paradise Lost", "The Way of the Warrior", "The Changing Face of Evil") This armor was first seen in use protecting an unshielded Defiant against Klingon disruptor fire. Throughout many conflicts in the 2370s, the armor protected the Defiant from, among others, the Klingons, the Cardassians, the Dominion and even from Federation phaser fire. During the failed coup attempt by Admiral Leyton, Captain Erika Benteen explicitly credited the Defiant's ablative armor with thwarting attempts by the USS Lakota to disable and prevent the Defiant from reaching Earth. (DS9: "Paradise Lost") By 2374, ablative armor had been installed on at least one other advanced Starfleet combat vessel, the USS Prometheus. (VOY: "Message in a Bottle")
It is of note that under certain conditions, chroniton particles generated by Romulan cloaking devices can become lodged in the ablative armor matrix. On at least one occasion, a temporal surge caused by the explosion of a microscopic singularity shifted the chroniton particles in the hull into a high state of temporal polarization, this caused a transporter beam to be redirected through time. This caused Commander Benjamin Sisko, Doctor Julian Bashir, and Lieutenant Jadzia Dax to be sent back in time to 2024, just before the Bell Riots. (DS9: "Past Tense, Part I")
The Borg's long-range tactical vessels were also equipped with ablative hull armor. (VOY: "Drone") The Hirogen outpost, designed as a training facility by means of holodeck technology, utilized three layers of ablative armor to protect the grid's holoemitters. (VOY: "Flesh and Blood")
Background
The Star Trek: Deep Space Nine Technical Manual clarifies what ablative armor is by describing it as an armor designed to disintegrate at a controlled rate under directed energy blasts, providing an additional layer of defense for starships.
During the late 20th century, spacecraft possessed a crude, in Star Trek terms, ablative material, which is defined as "any material that possesses a capability for rapidly dissipating heat from a substrate," and thus is descriptive of any type of armor or physical shield that employs that method of energy dissipation. Heat shields are constructed of ablative material.
It is unknown if or how this technology is related to the ablative generator-deployed hull armor, brought back from 26 years in Voyager's future and used by the USS Voyager in VOY: "Endgame".
Apocrypha
In Star Trek: Voyager - Elite Force, the Hazard Team's suits are equipped with ablative armor. While Munro (the player character) still has armor, damage to the health meter is greatly reduced. The Health Energy Stations present in the game use nanites to repair the armor.