A Geass (ギアス, giasu?) is a psychic ability that serves one distinct function, granted by individuals as such C.C. and V.V.
Geass is a mysterious ability which certain people can bestow upon others; C.C. is the most prominent character who grants the power of Geass. She calls Geass the Power of the King (王の力, Ou no Chikara?), and though she is unable to use the power herself, she is immune to the power being used on her. However, people who have Geass are not immune to other Geass' effects. After Lelouch confronts Mao, C.C. reveals the Geass manifests differently in each individual, possibly related to their inner desires. The power of Geass increases with each usage, and should the user lack the willpower, they may be consumed by it, which was the case with Mao. However, one who can resist and fully control the power has the qualities to become the king of all.
Every Geass, while powerful, has its own unique set of restrictions, limitations, or idiosyncrasies. These factors allow a Geass to be defeated, or its power limited, to someone who is aware of its characteristics.
All Geass abilities that have thus far appeared within the canon of the television series have been in effect related to the mind, influencing such things as will, thought, memory, and perception. Geass abilities in the manga sidestory Knightmare of Nunnally have no such limitations.
The power of Geass increases with use, usually starting in one eye. Most forms eventually spread to both eyes and become uncontrollable with repeated use. At this point, the one who granted the power can relinquish his or her immortality to the recipient, allowing the recipient to continue the cycle—forcibly if the giver desires—while the giver is allowed to die. The immortality is referred to as the giver's "Code," and grants the person immunity to Geass at the cost of their original power.