Geass (ギアス, giasu?) is a mysterious ability which certain people can bestow upon others; C.C.
is the most prominent character who grants the power of Geass. The form the Geass takes is different in each individual. C.C. calls Geass the Power of Kings (王の力, Ō no Chikara?), and is represented by a glowing, bird-like symbol.
Every Geass has its own unique set of restrictions, limitations, or idiosyncrasies. These factors allow a Geass to be defeated, or its power limited, by someone who is aware of its characteristics. All Geass abilities are related to the mind, influencing such aspects as will, thought, memory, emotion, and perception.
The power of Geass increases with use, usually starting in one eye. It can 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 while the giver is allowed to die. The immortality is referred to as the giver's "Code," and grants the person immunity to Geass in exchange for their original power.
"Geass" may be an intentional corruption of the word "geas" or "geis", a term for a type of magical contract in Irish mythology.