A physical representation of the gods.
The Gods is the term referring to the "collective human unconsciousness," which is the source of the individual identities for all humans, living and dead throughout the Code Geass Universe. Due to their human nature of not being equal, they are constantly in conflict with each other. Because of this, Charles Zi Britannia, Marianne Vi Britannia, C.C. and V.V. formed a contract with each other to destroy the gods and they were using the Sword of Akasha to that end.
Originally, references to the gods were considered some sort of metaphor by people such as Cornelia, but that has become less likely as the show progresses. It is later revealed that they are a collective subconsciousness of all humans beings metempsychosed into the World of C, which is visually similar to Jupiter. Charles intended to use the Sword of Akasha to trigger Ragnarök, that would destroy the gods and once destroyed, everyone would exist as one person. Lelouch, realizing that such a world would be stagnant, denies their attempt to create a new world. He used his Geass to influence the gods to turn against his parents and destroy them, thwarting Ragnarök from occurring.