Shield generator
From Memory Alpha, the free Star Trek reference.
A shield generator is a piece of equipment which generates some type of electromagnetic barrier or force field called a shield, designed to deflect kinetic or electromagnetic energy.
A cloaking device ties into the deflector shield grid of a ship, rendering it unable to shield while cloaked. (TOS: "The Enterprise Incident")
In 2372 during the Klingon attack on Deep Space 9, Gowron ordered Martok to concentrate all fire on DS9's shield generators. They destroyed at least two of them, which was reported by Dax later. (DS9: "The Way of the Warrior")
During the Second Battle of Deep Space 9, the Dominion destroyed Deep Space 9's shield generator. (DS9: "Call to Arms")
The shield generators on the USS Defiant were configured differently from those on its predecessor, in order to improve protection against the Breen energy dissipator. (DS9: "The Dogs of War")
One, an advanced Borg drone with 29th-century technology, modified the shield generators of the USS Voyager in 2375, so that they could break free of a Borg sphere's tractor beam. (VOY: "Drone")
[edit] Background
The phrase "shields up," which saw widespread use in Star Trek, probably had its origin in classical warfare, when infantry would raise their shields to defend against incoming arrows or other projectiles. The order would be given when it was time for the soldier to raise his shield from the carry position.
