Ablative armor is a secondary or tertiary defense mechanism for starships after shields. When hit by directed energy weapons-fire, this type of armor plating disintegrates incrementally. (Star Trek: Deep Space Nine Technical Manual)
Starfleet developed this technology sometime prior to 2371, with the USS Defiant being amongst the first Starfleet ships to have this technology installed. Ablative armor protected the Defiant against attacks by the Klingons, Cardassians, Dominion and even rogue Federation starships. (DS9: "Past Tense, Part I", "Paradise Lost", "The Way of the Warrior", "The Changing Face of Evil")
- In a possible future, Starfleet developed the technology further. Aboard starships in that version of 2404, armor generators on the hull distributed ablative armor to cover the entire hull. This proved to be extremely effective against Borg, Klingon and other hostile weapons fire. With the ship covered, however, phasers could not be used, but photon and other torpedoes could be fired.
A future Janeway brought an improved form of ablative armor technology back in time to be used aboard the USS Voyager. The technology was successfully adapted and used to get back to Earth; however, the technology has not been seen in use since on standard Starfleet vessels. (VOY: "Endgame")
The Star Triangle Advanced Guard improved the future ablative armor technology so that it could be used with standard shields and phaser arrays. This was achieved through the use of additional power systems, and a selective deactivation system similar to the one which allowed phasers to be fired through conventional shields. (Adventures of STAG)