Ablative hull armor

From Memory Alpha, the free Star Trek reference

Ablative hull armor is a Starfleet vessel defense technology developed sometime between 2378 and 2404.

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 heavy cruiser without any loss in armor integrity.

The armor is deployed via hull-mounted ablative armor generators. Though phasers cannot be utilized while ablative armor is deployed, apertures at torpedo launchers allow for the use of torpedoes.

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")

[edit] Background Information

• This armor should not be confused with the ablative armor used on the USS Defiant and USS Prometheus. There was no explicit verbal differentiation between the two technologies mentioned in "Endgame". It could be speculated that the technology seen in "Endgame" is an outgrowth of the ablative armor used in the "current" timeline.
• In a concept drawing of Admiral Janeway's shuttle by Rick Sternbach in 2001 for the final Voyager episode "Endgame" the hull armor is described as 'Nanotech Molecular armor plating'.
• Although not explicitly described on screen, it seems that the armor is deployed using a combination of replicator and transporter technology.
• The total volume of the deployed armor is substantial (not only does it cover the entire ship but its thickness is apparent as the plates materialize.) If the material is stored somewhere within the ship prior to deployment then the material would have to be compressible to minimize the storage space required. This would also mean that the density of the armor in its deployed form is comparatively low.
• It is unclear what happened to the ablative hull armor technology after Kathryn Janeway brought the armor back to the 24th century. Presumably it was confiscated by Temporal Investigations, as it was not seen in use on the USS Enterprise-E a year later. (Star Trek Nemesis)