The only known moon of the distant dwarf planet Eris. The name Dysnomia derives from the Greek for 'anarchy' - in Greek mythology, Dysnomia was the daughter of Eris ('strife'). Before being officially named, Eris and Dysnomia were unofficially known as Xena and Gabrielle.*
Dysnomia is notable for its significant size. Its diameter is approximately 700km, and after Pluto's moon Charon, Dysnomia is the second largest known satellite of any dwarf planet or asteroid. This makes it about a third of the diameter of Eris (itself one of the largest of the dwarf planets) which Dysnomia orbits at a distance of some 37,350km, in a period of a little less than sixteen days.
Dysnomia was likely formed in an ancient collision, coalescing from material broken away from Eris (the fact that moons are often found around the larger dwarf planets suggests that such collisions were commonplace in the early Solar System). Little is known about Dysnomia's physical structure, except that it has a dark surface that reflects very little light back into space.
* Xena and Gabrielle were characters from the television series Xena: Warrior Princess, and the name Dysnomia maintains a tenuous but intentional connection to that original source. One interpretation of the name is 'lawlessness', connecting the moon with the actress Lucy Lawless, who portrayed Xena in that series.