The large area of Middle-earth east of Rhovanion stretching from the eastern edge of Mordor north to the Iron Hills.

The inhabitants of Rhûn were considered Easterlings and were mostly loyal to Sauron, fighting against Gondor and its allies.

In TA 1856, an army of Wainriders out of the east of Rhûn went to war against Gondor and took control of all of Rhovanion, enslaving the inhabitants. They continued to war with Gondor until TA 1944 when they were finally defeated at the Battle of the Camp.
Encyclopedia entry originally written by PotbellyHairyfoot