American Tract Society Bible Dictionary 
Descendants of Midian, a nomade race in Arabia, numerous, and rich in flocks, herds, and camels, Isaiah 60:6 . The original and appropriate district of the Midianites seems to have been on the east side of the Elantic branch of the Red Sea, where the Arabian geographers place the city Midian, Acts 7:29 . But they appear to have spread themselves northward, probably along the desert east of Mount Seir, to the vicinity of the Moabites; and on the other side, also, they covered a territory extending to the neighborhood of Mount Sinai. See Exodus 3:1 18:1 Numbers 22:25,31 Judges 6:1-8:35 . In Genesis 25:2,4 , compared with Genesis 25:12-18 , they are distinguished from the descendants of Ishmael, though elsewhere we find the two people intimately associated, so that they are called now by one name and now by the other. See Genesis 37:25 , compared with Genesis 37:36 . Their capital city was called Midian, and its remains were to be seen in the time of Jerome and Eusebius. It was situated on the Arnon, south of the city Ar, or Areopolis.
The Midianites were idolaters, and often led Israel astray to worship their gods. They also not infrequently rendered the Hebrews tributary, and oppressed them. See Numbers 22:1-41 25:1-18 31:1-54 . Often when the Israelites had sown, and their harvest was nearly ready to be gathered in, the Midianites and Amalekites, children of the eastern desert, came down like locusts in countless swarms, with their cattle and tents and camels, to devour and carry off the fruits of the ground, and not only rob but destroy their owners. And often did the Jews, lacking the strength or the faith or the leadership necessary for effectual resistance, seek refuge in mountain-dens and caverns till the invaders retired. Gideon was their deliverer in one such period of oppression, Judges 6:7 . The modern Ishmaelites still follow the ancient practice, and their violent incursions, robberies, and murders might be described in the same terms that were used with reference to their fathers by the historians of old.
Kitto's Popular Cyclopedia of Biblial Literature 
Mid´ianites, a tribe of people descended from Abraham's son Midian. His descendants must have settled in Arabia, and engaged in trade at an early period, if we identify them with those who in the time of Jacob appear, along with the Ishmaelites, as merchants traveling from Gilead to Egypt, and who, having in their way bought Joseph from his brethren, sold him in the latter country . It is, however, very difficult to conceive that the descendants of a son of Abraham, born so many years after Isaac, had become a tribe of people at the time when the descendants of Isaac himself were so few. One is therefore much inclined to suppose that these Midianites were different and distinct from those descended from Abraham's son; and there appears the more ground for this when at a later period we find two tribes of Midianites, different in locality and character, and different in their feelings towards the Israelites. If this distinction be admitted, then it would be necessary to seek the earlier Midianites in those dwelling about the eastern arm of the Red Sea, among whom Moses found refuge when 'he fled from Egypt,' and whose priest or sheikh was Jethro, who became the father-in-law of the future lawgiver (;; ). These, if not of Hebrew, would appear to have been of Cushite origin, and descended from Midian the son of Cush. We do not again meet with these Midianites in the Jewish history, but they appear to have remained for a long time settled in the same quarter, where indeed is the seat of the only Midianites known to Oriental authors.
The other Midianites, undoubtedly descended from Abraham and Keturah, occupied the country east and south-east of the Moabites, who were seated on the east of the Dead Sea; or rather, perhaps, we should say that, as they appear to have been a semi-nomad people, they pastured their flocks in the unsettled country beyond the Moabites, with whom, as a kindred, although more settled tribe, they seem to have been on the most friendly terms, and on whose borders were situated those 'cities and goodly castles which they possessed' . These Midianites, like the other tribes and nations who had a common origin with them, were highly hostile to the Israelites. In conjunction with the Moabites, they designedly enticed them to idolatry as they approached Canaan ; on which account Moses attacked them with a strong force, killed all their fighting men, including their five princes or emirs, and made the women and children captives (Numbers 31). The account of the spoil confirms the view which we have taken of the semi-nomad position of the Midianites—namely, 675,000 sheep, 72,000 beeves, 61,000 asses, 32,000 persons. This was only the 'prey,' or live stock; but besides this there was a great quantity of 'barbaric pearl and gold,' in the shape of 'jewels of gold, chains, and bracelets, rings, earrings, and tablets.'
Sometime after the Israelites obtained possession of Canaan, the Midianites had become so numerous and powerful, that, for seven successive years, they made inroads into the Hebrew territory in the time of harvest, carrying off the fruits and cattle, and desolating the land. At length Gideon was raised up as the deliverer of his country, and his triumph was so complete that the Israelites were never more molested by them (; Judges 7; Judges 8). To this victory there are subsequent allusions in the sacred writings ; but the Midianites do not again appear in sacred or profane history.
The Nuttall Encyclopedia 
A race of Arabs descended from Abraham by Keturah, who dwelt to the E. of Akaba; though related, were troublesome to the Hebrews, but were subdued by Gideon.