Midian is the name of a son of Abraham and Keturah. He and his brothers were sent by their father to the east of the Jordan River (Genesis 25:1-6). Of all the brothers, only the name Midian continued to exist, suggesting that he was the most powerful of them, and that the descendants of the others assimilated with the Midianites. Only the names of Sheba, Dedan and Efa recur in biblical history.
At the time of Moses the Midianites had their own territory (Exodus 2:15). When the people of Israel went up to the promised land, they met with Midianites near Moab. The Midianites were causing problems, and God told Moses to take revenge (Numbers 31:1-12). At another time the king of Edom fought against the Midianites in Moab and defeated them (Genesis 36:35).
At the time of Gideon, the Midianites attacked Israel several times and dominated them for several years.
It seems the Midianites first lived in a large area stretching from the east coast of the Red Sea to Moab. The victory of Gideon over the Midianites seems to have been so great, that they were driven back to the area along the Red Sea, southeast of Aqaba.
The people never recovered and are no longer mentioned in the Bible, except for references to the times of Moses and Gideon (Psalm 83, 1 Kings 11:18, 1 Chronicles 1:46, Isaiah 9:4, 10:26, Habakuk 3:7).
The only exception is Isaiah 60: 6, where the prophet predicts that Midian will serve God. This wonderful promise has not been fulfilled, so it still remains to take place.