Geass

Geass (ギアス, giasu, referred to as The Power of The King by C.C.) the namesake of the Anime, is an unusual and seemly supernatural ability that gives the user certain powers over the human mind and body. Each person's Geass is unique, with specific powers and limitations. Actually using the Geass involves a bright purple bird- or "V"-shaped sigil glowing in one or both eyes, and is represented in the Anime as a bright, expanding field of energy.

Origin
The word Geass seems derived from the word geis, a term for a magical contract in Irish/Celtic Mythology, usually in which a person rewarded for obeying the terms of their contract, but doomed for violating it, often regarding to a persons fate or prophecy. Just as in its original form, Geass in the Code Geass universe seems to only be obtainable through a contract with witch-like humans such as C.C. and V.V., and this contract each comes with a certain price, in the case of the Emperor, to destroy the gods.

Lelouch's Geass
Lelouch Lamperouge has the power to give any order and have it followed, given to him by C.C.. While it can be used on multiple targets simultaneously, Lelouch must have eye contact with his targets for them to be affected. While this will not work through a camera or Knightmare's sensors, it can be reflected, such as with a mirror. People who he uses the Geass on will have no memory of being Geassed, or while following orders. There is no time limit for how long a person will follow orders, and a person may be commanded to only do something at a far distant time, remaining free from Geass influence until then. Lelouch's Geass only works one time per each person, and all commands must be given verbally.

Lelouch's Geass became permanently active in the first season, and when it is is re-activated by C.C., he uses a contact lense to hide the glowing sigil in his left eye.

Mao's Geass
Mao, prior to his death, had a very powerful Geass that allowed him to read minds of all people within a 500 meter radius, or a single person if he concentrates. He was given his Geass by C.C. as well, but she apparently abandoned him as he was unable to fulfill the contact. His Geass eventually becomes permanently active, and the constant influx of thoughts drives him insane.

Charles's Geass
Charles Di Britannia is shown, in the begging of the Second Season, to have a very powerful Geass that can completely rewrite memories, which uses on the captured Lelouch - believing he still has a use for him - to return him to a modified version of his old life - and a new brother. He also has the apparent ability to deactivate another Geass users Geass, and may even have to ability to steal another's Geass. This Geass was given to him by his brother V.V., as part of a contract to destroy the gods.

Rolo's Geass
Rolo Lamperouge has the ability to induce a complete paralysis of the body and mind of all people withing a certain spherical area, called the 'the seal of absolute suspension', and making it seem like he has the ability to teleport (as the affected feel no passing of time while frozen). The extent of this area can be controlled, though its maximum limit is unknown, does not require eye contact, and. Rolo's heart stops when he uses the Geass, which is his main limitation, both it time (usually around ten seconds) and the number of people affected. He was presumably given this Geass by V.V.

Other Geasses
When the Black Knights invade the Geass Directorate, an un-named Geass-capable child is shown taking control of a Knightmare pilot's body and forcing him to attack a comrade, though the remains self-aware and tries to resist, shouting out that he is not responsible.

Geass Canceler
Jeremiah Gottwald has the ability to cancel or null the affects of any other Geass, both for himself an other people. This includes all forcibly-forgotten or rewritten memories, and even The ability, like Rolo's, seems to expand spherically, and may affect people well out of sight, and can remove the affects of multiple Geassings as once. Since the Geass originates from the bionic eye of Jeremiah's face, and is blue rather than purple in color, is is genernally assumed that it is not a true contract Geass, but a Geass-derived technology, given to him in addition to his other bionic body parts by the Geass Directorate. Those who have been affected by one-time-only Geasses (like Lelouche's) may be affected by that Geass again once it has been annulled.