Nix (somtimes called Pluto II), along with Hydra and Charon, is one of three known moons of the dwarf planet Pluto. It was discovered with Hydra in June 2005 by the Hubble Space Telescope. It is named for Nyx, the Greek goddess of night and mother of Charon, though the Egyptian spelling of the name is used to distinguish it from the asteroid 3908 Nyx. Before a name was decided on, Nix was provisionally dubbed S/2005 P 2. It is the middlemost of Pluto's three moons, and an animation of its place in relation to Pluto's other satellites can be found here.
It is a great deal smaller than either Pluto or Charon, its radius likely somewhere between 20-70 km. Nix was probably formed from the same large impact astronomers speculate resulted in Charon.