Shield frequency

A shield frequency is the frequency which the shield emitters are set to generate the shielding on a starship or other shielded location.

The Borg use regenerative shields which constantly remodulate their shield frequencies to best resist enemy fire. (TNG episodes: ''Q Who?, The Best of Both Worlds, et al.)

An unusual hybrid starship encountered by the USS Enterprise-D in 2370 included Borg components and operated with a Borg shield structure, the constant shift in shield frequencies made it hard to gain a proper sensor lock on the vessel to learn anything about it. (TNG comic: Strategy)

If an attacker is aware of the shield frequency of a given ship they can adjust their weapons frequency accordingly to fire through the shields, this tactic was used in 2371 in a devastating attack on the USS Enterprise-D. (TNG movie: Generations)