The Sea of Chaos (Japanese: 混沌の海 Romaji: konton no umi, Runic: ᚴᚾᚫᛋ Chaos) is what surrounds/exists outside of the four physical sides, or worlds, in the Slayers and (depending on the version of the series) the Lost Universe multiverses. These worlds are sometimes described as standing on the Sea of Chaos, or having risen from it; however, these are merely explanations of a phenomena which takes place outside of the three-dimensional world we can comprehend. The Sea of Chaos is both the embodiment of utter chaos and complete void, and as such is none other than the Lord of Nightmares herself.
Because the Giga Slave summons the Lord of Nightmares, it is the only spell known to be capable of opening a "gateway" between the Sea and the physical worlds. The only beings known to have entered the Sea of Chaos this way (or entered it at all), aside from the Lord of Nightmares, are Lina Inverse and Gourry Gabriev.
Chaos is one of the names listed in Runic on the Plane chart.