Geass also referred to as the Power of Kings is a mysterious ability which certain people can bestow upon others (the word itself potentially being an intentional corruption of geas or geis, a type of magical contract in Irish mythology). According to an English edition of Newtype, the power of Geass has something to do with the very existence of humankind and it may be used to destroy or transform just about anything.[10] It is represented by a bird-shaped symbol which glows red when active.

A Geass is set as a contract between an immortal and a human who receives a unique power. It manifests in the eyes, and with continued use increases in power, eventually becoming uncontrollable and spreading to both eyes. When a Geass is at full power, a person can then become a recipient of the immortal's "Code", that which grants them their immortality. This new immortal receives the Geass mark somewhere on their body, immunity to Geass, and the ability to make their own contracts with others, while the giver is allowed to die (perpetuating an endless cycle). This process is not necessarily consensual, as C.C. received her Code and V.V. had his taken away against their wills.

Every Geass power has its own unique set of both abilities and limitations (for example, Lelouch's power, the ability to make anyone obey him, can only be used once on a given person), allowing for their defeat or victory by someone who is aware of its characteristics. According to C.C. in the television series, the way geass manifests itself in a person is determined by the person's deepest/truest desire (i.e. C.C.'s geass made everyone love her in response to her desire to be accepted and loved). Further understanding of the power of Geass even leads to the development of a device that can undo its effects. All Geass abilities that have thus far appeared within canon of the television series have been related to the mind, influencing things such as emotion, perception, and memory.