Installation of ablative generator on Voyager.
USS Voyager deploying ablative hull armor
Shuttlecraft SC-4 with armor deployed
An ablative generator was a type of Starfleet technology that was in use by the early 25th century. The ablative generator was designed to protect starships by generating a nearly inpenetrable hull armor, which was deployed from the externally mounted generators, that when deployed expanded across the ship's hull. Though phasers could not be utilized while ablative armor was deployed, apertures at torpedo launchers allow 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.
The armor was first seen used by an alternate timeline version of future Admiral Janeway on her shuttlecraft while escaping the Klingon Korath in 2404. Admiral Janeway then traveled to 2378 and provided the armor technology to the crew of USS Voyager in order for them to penetrate the defenses of one of only six heavily-defended Borg transwarp hubs. It could barely be penetrated by disruptors or stop the ship with tractor beams. (VOY: "Endgame")
Background
After its critical role in "Endgame", the armor was not shown or mentioned in any film or series again.
This armor should not be confused with the ablative armor used on the USS Defiant and USS Prometheus. Concept drawing of Admiral Janeway's shuttle by Rick Sternbach described the future armor as Nanotech Molecular armor plating.
According to Mr. Scott's Guide to the Enterprise (page 14), the deflector shields of the refit-Template:ShipClass were a similar technology, while only the defense fields protecting the top decks were traditional force fields. The main shield grid was a massive replicator system, projecting an invisible hull layer, moleculary identical to the diburnium-osmium alloy used to build the Kalandan outpost.