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 had some of the most powerful shields ever created. Their shields were able to withstand multiple hits from energy and projectile weapons for some time. The most powerful Ancient shield was the shield of Atlantis. Powered by a ZPM, they were capable of withstanding Wraith energy weapons and even the weight of the ocean on Atlantica. During a storm year, Rodney McKay and Radek Zelenka used lightning strikes form the storm to power the shield, though they were unable to activate the shield until the ZPM arrived.

Asgard
The Asgard's shields are equally impressive. They are capable of resisting most shots from a Ha'tak, as well as the central laser weapon on an Ori battlecruiser. While travelling through hyperspace, Asgard shields are inactive, leaving them vulnerable to attack. Samantha Carter used this tactic against a Replicator fleet once.

Goa'uld
The Goa'uld have shields which are impressive, though they are not comparable to the Ancients or the Asgard. The shields carried on ha'taks are capable of resisting naquadah enchanced nuclear warheads, and can remain in a coronosphere for up to ten hours without the crew feeling effects. 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 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 energy of an activating Supergate.

Wraith
Though hive ships and Cruisers do not appear to employ shields, Darts use shields to serve as canopies for their cockpits. These shields are completely opaque, and send infromtation to the pilot in written Wraith.