Regenerative shield
From Memory Alpha, the free Star Trek reference.
Regenerative shields are an advanced shield system used by the Borg and other species.
The basic principle of regenerative shielding is to quickly analyze an attacking weapon's frequency and then modulate shield freqency to increase damage mitigation without depleting shield energy.
Regenerative shields are used on Borg vessels and Borg drones. When in combat, Borg drones can be killed by phasers but only for the first several shots. The phaser becomes ineffective if used at the same setting after multiple shots. The phaser's effectiveness can be extended though by adding a frequency modulation chip. (TNG: "Q Who", "The Best of Both Worlds", "The Best of Both Worlds, Part II") However, Borg drone sheilds appear to be less resilient to Projectile weapons; when the Borg attempted to assimilate the Enterprise-E in 2063, Jean-Luc Picard lured two drones into the holodeck area and used a holographic Thompson Submachine Gun to easily kill the two drones.(Star Trek: First Contact)
Starships from other races equipped with this type of shielding include sensors integrated within the shield systems' own energy that alters the frequency to better protect the ship.
The battle systems of Starfleet's Prometheus-class starships include regenerative shielding, ablative armor, and multi-vector assault mode. (VOY: "Message in a Bottle")
The warships of the Numiri are protected by regenerative shields. (VOY: "Ex Post Facto")
