Mizar II was the inhabited second planet orbiting the primary Mizar. This was the homeworld for the Mizarians, a warp-capable pacifistic humanoid species.
Due to the Mizarians' traditions of peace, the planet was conquered six times in a matter of three centuries. (TNG: "Allegiance")
This planet was only mentioned in dialogue.
According to Star Trek: Star Charts (p. 44) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), the Mizar (Zeta Ursae Majoris) system was located in the Alpha Quadrant. It was a quinary system of five A-class stars. In the late 24th century, it was an independent system.
According to StarTrek.com, the Mizar system was located about 88 light years from the Sol system.
In a line cut from the filmed episode, Tholl stated that the third-largest city on Mizar II was Pozaron.