Deflector shields

The deflector shields (or more commonly referred to simply as shields) are an electromagnetic force field that can be projected around a starship, starbase, or planet by technological means and act as the first form of defense against enemies or natural hazards, using deflection to protect said body from various forms of radiation and matter collisions.

Description
Most starships are fitted with deflector shields, often projected from a deflector dish, to protect them from particle impacts which, particularly at high velocities, could cause huge amounts of damage. While basic deflectors and deflector beamscould provide minimal protection from weapons fire and basic mishaps, the majority of advanced starships were also fitted with more powerful shielding which could be activated in tactical situations. However, these shields were sufficiently powerful that they also blocked out transporter beams.

Shields could be configured in two forms; ellipsoid form shielding, common in Federation starships in the 24th century, which projected a shield bubble around a starship; or contour-conforming shielding which was projected much closer to the ship's hull, providing more specifically localized protection. Some shielding systems also allowed shielding over specific areas of a ship to be activated or deactivated to provide protection for a specific area only and presumably reduce power consumption not protecting the entire hull.

Horta were seemingly invulnerable to even very powerful shields, being capable of passing straight through them with very little effort.

While conducting operations in the Badlands in the early 2370s, the Maquis discovered a way to apply regenerative tuning techniques in order to keep their shields operative in the extreme magnetic and radiation flux of the region. In early 2372, the USS Enterprise-E applied some of these techniques to their shields while inside the Pantera Nebula.

In addition to standard deflectors, there a number of other shielding technologies that were developed including:
 * Adaptive shielding used by the Borg, which analyze weapons fire and adjust shield frequencies to better resist outside forces. This type of technology appears to rely upon the Collective's considerable processing power to analyze incoming weapons fire. (TNG episode: "The Best of Both Worlds", et al.)
 * Metaphasic shields, developed by the Ferengi Dr. Reyga, are capable of withstanding the pressure, radiation and energy of a star's corona. (TNG episode: "Suspicions")
 * Covariant shielding was an extremely powerful form of starship deflector shielding technology, difficult to penetrate with conventional weaponry. (VOY episode: "Tsunkatse")
 * Regenerative shielding  was used on the highly advanced Federation tactical cruiser USS Prometheus. It utilized multi-layered/tier shielding that enabled rapid recharging (regeneration) of resting layers, so if the initial layer taking the brunt of an attack gets depleted another fully charged layer is ready to take over while the initial one recovers.
 * Unimatrix shielding used on the Delta Flyer, was a Borg influenced technology, that utilized Borg algorithms to help optimize shield frequency against incoming attacks. (Although not granting the same level of adaptive frequency invulnerability as true Borg shields, it did however impressively enhance conventional shield defensive strength).

Frequencies
257.4 was the shield frequency of the USS Enterprise-D in 2371. The Klingon Duras sisters had previously captured Geordi La Forge and while under their captivity had Tolian Soran (a scientist they were working with) modify La Forge's VISOR to transmit a video stream to them. When Geordi returned to the ship, the sisters were able to expose the frequency and fire through the Enterprise's shield.

Earth and Federation Shields
By the 22nd century, humans had only developed basic deflector dish technology, which allowed them to operate more safely at warp speeds, but had to resort to polarized hull plating in tactical situations. Many species in the Alpha and Beta Quadrants had developed more advanced shielding technology by the 2150s, these included the Andorians and the Klingons.