Reaver space (also known as Reaver territory) was an area of the the 'Verse that the Reavers controlled. It was dominated by a floating field of Reaver vessels in an area near the mysterious planet Miranda.
Malcolm Reynolds once traversed Reaver space in the hopes of finding Miranda by altering his ship to look like one of the Reavers', he was successful in this attempt. As he departed Miranda, he traversed this space a second time. This time, he fired upon a crab-shaped vessel to provoke the Reavers into pursuit. As planned, they followed Serenity to a large fleet of the Union of Allied Planets Navy, forcing both fleets into the Universe battle.