Shield

Shields are devices which use energy that prevents matter from passing through them. Shields are used by many races, so they can vary.

Ancients
The Ancients possessed some of the most powerful shields ever created, capable of withstanding multiple hits from both projectile and energy weapons for a lengthy amount of time, and extreme natural phenomena. The most powerful shields ever built were ancient, the shields of City-ships. Powered by ZPMs, they are capable of withstanding Wraith energy weapons, the weight of the ocean on Lantea for 10,000 years, and even coronal mass ejections. During a storm year, Rodney McKay and Radek Zelenka channeled the energy of the storm's lightning strikes to power the Atlantis shield, although when the Wraith attacked they were unable to activate the shield until a ZPM arrived. Unlike the shield technology of other space-faring races, the effectiveness of the shield of Atlantis does not deplete under external forces, and will remain impenetrable as long as sufficient power exists to sustain it. This means that provided there is a sufficiant power source, the shield in invincible. The more stress the shield is under, the more power it uses, so with one ZPM it can survive under the ocean for 3.3 thousand years but only last a few days against an Asuran Stargate satellite weapon (also ZPM powered.)

Aurora-class warships possess shields not seen on any other ship. The shields equipped on the Aurora-class are contoured around the hull of the ship, instead of just forming a 'bubble' like other shields do. Thease shields are the most common amongst ancient ships and do depleate over time like other races shields.

The Ancients also created a personal shield. Once activated by someone with the ATA gene, this shield would protect it's user from anything from gunshots, to falling off a balcony and even powerful explosions. Its one weakness was that the user needed to breathe, so they were still susceptible to things such as drowning. A fully powered shield has yet to be seen to be depleated by weapons fire.

Asgard
The Asgard shields are equally impressive to the ones on the Aurora, capable of resisting most shots from a Ha'tak, as well as several consecutive hits from the central laser weapon on an Ori battlecruiser, which has never been done before. However, while traveling through, and for a short period of time after exiting, hyperspace, Asgard shields are inactive, leaving their ships vulnerable to attack. Samantha Carter would exploit this weakness to develop a tactic against a Replicator fleet converging on the Asgard homeworld. In gratitude to the Tau'ri for their help against the Replicators, the Asgard gave them access to much of their technology, including shields. The shields on Tau'ri vessels are not as powerful as those on Asgard ships, but that is only due to the power requirements. Another weakness of Asgard shield technology is the fact that, not only is the effectiveness of their shields directly proportional to the amount of power regulated to them, they will also gradually weaken under sustained fire, allowing energy from attacks to bleed through, and so they need time to recharge afterwards. Asgard shields are also one of only two known types of shield technology capable of resisting direct penetration from drone weapons, allthough they do take serious damage from them.

Goa'uld
The Goa'uld have shields which, while no doubt impressive, are not comparable to the shields of the Ancients and the Asgard. The shields carried on Ha'taks are capable of resisting naquadah enhanced nuclear warheads, and can remain in a chromosphere of a star for up to ten hours without the crew feeling effects. Shields are also used to cover the viewing post on the pel'tak, as the Goa'uld have no transparent substance capable of withstanding hyperspace. Goa'uld shields are directly proportional to the amount of kinetic energy used, meaning it will stop energy weapons and high speed projectiles, but not slower moving projectiles.

Ori
The Ori possess some of the most powerful shields known. They are capable of withstanding virtually anything, including a fleet of ha'taks, Asgard energy weapons, even the Dakara superweapon. Bra'tac once tried to crash a fully functioning ha'tak into an Ori battlecruiser at full speed, but it merely impacted within the shields. The only thing known to overwhelm the shields of an Ori battlecruiser is the unstable vortex of an activating Supergate, and the most advanced Asgard beam weapons. In an alternate reality, Samantha Carter is told that a single Ori battlecruiser was fought off by drones controlled by the Ancient contol chair in Antarctica. Whether this means that Drones are capable of directly penetrating an Ori shield was unknown. The Ori also possesses shields which can collapse a planet into a micro-singularity. These shields expand after absorbing enough energy, which can be accelerated by weapons fire, allthough a lot of energy was sent through the Stargate to allow that.

All Ori shields are built with Ascended knowledge

Tau'ri
The Tau'ri do not possess shields of their own design, instead utilizing shields of Asgard design. However, when Rodney McKay had his intelligence increased he did pass on an algorith to Hermiod which would increase the effectiveness of shields on the Daedalus-class warships. Whether the algorithm was entirely McKay's design or was if he was utilizing Ancient shield technology is still unknown.

Tok'ra
The Tok'ra have one-way shields, usually arranged to enclose a space, that holds matter inside but allows external objects to enter. They have been used in an attempt to capture a Kull Warrior, and to keep Khalek imprisoned while allowing shots to be fired into the cell.

Wraith
Hive-ships do not have any energy shields per se, instead relying on an organic armored hull that has a limited form of regeneration to nullify damage. Hive-ships do, however, have a program which prevents beaming technology from beaming objects inside the ship or close to it's exterior. Cruisers do not appear to have shields of any kind, while Darts use shields to serve as canopies for their cockpits. These shields are completely opaque, and send information to the pilot in written Wraith.