Shield Generator

A shield generator, or shield emitter, was the physical component of starships, space stations, and other installations, utilized for the generation of protective energy screens with which to defend against outside attacks. In combat situations, it was common for a vessel to target weapons fire at an adversary's shield generators, in an attempt to disable or weaken their shield strength. It is also common for additional power to be redirected to the generators to bolster shield strength in times of danger. Weakened shields can be recharged, but only after they are taken offline and placed on a recharge cycle. This has to be done in an area of relative safety, as the ship or station will be vulnerable.

The Federation utilized mercassium, a synthetic composite, in the constructions of their shield generators, a technology they did not share.

From Memory Alpha