Deflector shield
Deflector shields, screens, or shield grids, generally referred to simply as shields, were a type of force field that surrounded a starship, space station, or planet to protect against enemy attacks or natural hazards.

Operation 
Deflector shields operated by creating a layer, or layers, of energetic distortion containing a high concentration of gravitons around the object to be protected. On starships, the shield contained six sections, forward, starboard, port, aft, dorsal, and ventral. () Shield energies could be emitted from a localized antenna or "dish", such as a ship's navigational deflector, or from a network of "grid" emitters laid out on the object's surface, such as a starship's hull. Since at least the 23rd century, deflector shields were essential equipment on starships.

Neither matter nor highly concentrated energy could normally penetrate a shield. When shields were "up," or energized at a high level, most matter or energy that came into contact with the shields was harmlessly deflected away. This was important in starship combat, as shields were essential for hull protection. When the shields were up, only minor hull damage would be expected during combat. In the 23rd and 24th century, without deflector shields weapons were capable of causing catastrophic damage to starship hulls almost immediately. (; ; )

Continuous or extremely powerful energy discharges could progressively dissipate the integrity of a shield to the point of failure. Shield capacities varied according to many variables, from the power available to environmental concerns, making definitive and universal calculations of how much damage they could take difficult to estimate. Therefore, during combat tactical officers would continually report on shield strength, usually as a percentage of total effectiveness, with 100% meaning that the shields were at full capacity, and lower percentage scores indicating weaker shield conditions. 

Specific sections of the shield grid could take more damage than other sections, and be reinforced with additional power reserves, so tactical officers would report on the health of the shields by section if need be. Shields were said to be "holding" if damage was not sufficient enough to allow a compromise; if the shields were "buckling" or "failing," then a total loss of shield protection was imminent. ()

Shields operated within a range of frequencies to allow certain specific types of energy and matter to pass through, or to increase the effectiveness of blocking them. The frequencies of shields were not usually discernible without examining the controls on board the ship deploying them, meaning that it was very difficult to tune weapons to the exact frequency of an opponent's shields to bypass them. (; )

In combat situations, starships would match their own shield and weapon frequencies so their shielding did not interfere with their own weapons. Some weapons technologies, including those commonly used by the Borg, had rapidly adjustable frequencies, meaning that they could quickly penetrate a shield that was using a static frequency. An effective counter was to repeatedly and randomly alter the shield harmonics to minimize the effectiveness of the weapon's retuned frequencies, though this tactic amounted to little more than a stall against the Borg. (; )

It took time to activate a deflector shield. A refit  starship needed exactly 13.5 seconds to lower and raise its shields when taking a shuttlecraft on board via its tractor beam, though that included the time required to tractor in the shuttle during an automated docking procedure; flying the shuttle in manually reduced this time significantly. ()

Shields on a  starship could be fully recharged after charging the shield emitters for 45 seconds. This task required the shields to be dropped. ()

Normally, transporters were not capable of penetrating shields. () Twenty-third century Federation starships, such as the Constitution-class , could not transport through their own shields, but later starships such as the  vessels could transport personnel and objects freely to and from without having to lower their shields. ()

There were at least two different shield configurations commonly used by Federation starships. One type projected an ellipsoid shield bubble around the ship and a relatively large region directly adjacent to the ship. The other shield configuration used a contour-conforming shield layer projected a few meters outside the main hull.

Some starships had a shield system advanced enough to allow protection of only specific areas of the ship, leaving other areas unprotected. This was useful in times when power reserves are low. Lowering portions of the shields would allow the use of transporters without completely sacrificing the protection that shields provide. This was also useful if a captain wanted to leave a section of the ship unshielded, like a cargo bay. ()

Shields could be "extended" to encompass another vessel which couldn't use its own shields. () This could also be used on ships with weaker shields, by matching the ships' shield emitter frequencies. (; ; ) While the extended shielding could provide some protection in the event of a shield failure on one ship, this technique put a strain on that ship's systems, and the overall strength of the extended shields was generally not as strong as two independent shield systems.

Strength 
Shield strengths seemed to vary, making an exact calculation of how much damage they could take impossible. However, there were some examples:
 A Constitution-class starship's shields could take the equivalent of ninety photon torpedoes at once. They could do so a total of five times before shattering completely. ()
 A  starship could be destroyed by two photon torpedoes from a Galaxy-class starship with weakened shields. ()
 Transporters on a Galaxy-class starship could penetrate the shields on a  starship with weakened shields. ()
 The type of deflector shield used by the Lysian Central Command had an output of 4.3 kilojoules and could have easily been taken out with a single photon torpedo. ()
 According to Malcolm Reed, the shields of the 22nd century Nausicaan ships won't hold against a torpedo from . ()

Weaknesses 

During the first battle with the Dominion in 2370, the 's shields were useless against the attacking Jem'Hadar fighters phased polaron beams, therefore the ship suffered significant hull damage during the attack, even losing viability to its port nacelle. To compensate the Odysseys Captain Keogh attempted to alter the shield harmonics to compensate, but was unable to find an effective frequency, even after they had run through the full spectrum. Consequently, with the loss of their primary defensive mechanism, Keogh had engineering divert shield power to weapons to bolster their offensive capabilities. () Four years later, Weyoun 5 was surprised to learn that Deep Space 9's shields held against the Dominions initial assault during the second Battle of Deep Space 9. In fact, he called such a feat "impossible", adding, "Federation shields have always proven useless against our weapons." Nevertheless, he was reassured by Gul Dukat that "I've found it wise to never underestimate the Federation's technical skill or Captain Sisko's resourcefulness." ()

Uses 

In 2369, Starfleet made first contact with a species from the Gamma Quadrant known as the Hunters. The first contact ended in a shootout on the Deep Space 9 Promenade, wherein the Hunters used a handheld deflector shield, integrated into their uniforms on their left arms. These shields collected the phaser fire and bundled it, letting the Hunters take several phaser shots without being hit. ()

Later that year, Major Kira Nerys and Lieutenant Jadzia Dax re-calibrated the inner deflector shield subsystems of Deep Space 9. ()

On Nepenthe, the house of William T. Riker and Deanna Troi had been equipped with deflector shields and scanners by 2399, as Riker said they had been having "a little trouble... with the Kzinti." ()

In 3189, as the  descended to the surface of a planet, Acting Captain Saru ordered Lieutenant Joann Owosekun to activate the deflector shields in bursts so as to cushion their landing. ()

Shield types 
Auto-regenerative shielding
Covariant shield
Defense field
Immersion shielding
Metaphasic shield
Metaphysic shield
Multi-adaptive shield
Multi-spatial force field
Multi-spectrum shielding
Multiphasic shield
Paratrinic shield
Refractive shield
Regenerative shield
Structural integrity shield
Temporal shield
Unimatrix shield

See also 
Chromodynamic shield
Military technology

cs:Štítový systém
de:Schutzschild
es:Escudos
fr:Bouclier déflecteur
ja:ディフレクター・シールド
pl:Deflektor
pt:Escudo defletor
Shield technology