Slavery simply means the possession of a person as property. Slavery was a well-recognized social institution before the advent of the white man in the 15th century.
However, the origin of the trans-Atlantic slave trade in Nigeria could be traced to the middle 15 Century (precisely 1441). At that tim6, the Portuguese exported Africans to Lisbon and other parts of the world.
The middlemen traders along the coast were instrumental for the acquisition of the victims. The discovery of Americans in the 1470s gave rise to the expansion of the slave trade.
Moreover, the number of Nigerians that were shipped from the coastal region was not known but between the 16th and 18th centuries, it was estimated that about 24 million Africans were shipped from West Africa and Angola and that about 22,000 were victims from Nigeria every year.