Although the trans-Atlantic trade in slaves from the fifteenth to the nineteenth centuries has been the focus of many discussions concerning slavery, it is important to note that this period of forced migration was not a singular occurrence and that the use and movement of slaves have existed in various forms for millennia. The term "slavery" is problematic in itself, as the forms of slavery differ enormously from society to society. The most common definition of a slave is one who is the legal property of another or others and who is bound to absolute obedience. Large-scale slave systems, those in which slavery was essential to the social or economic structure, can be found in at least fifty-five societies throughout the Old World. The slave societies of ancient Greece and Rome, Visigothic Spain, Iceland, Russia, Korea, China, and Iraq, for example, all relied upon the labour of urban and rural slaves. Slaves could be given as part of a dowry, or acquired through birth, trade, warfare, capture, or voluntary servitude. Certain slave societies, such as that of medieval Ireland, for example, used slaves as forms of payment. The cumal, or seven years of labour by a female slave, was of equivalent value to three ounces of silver, or between eighteen and sixty-four cows. What is especially interesting to note is that in 75 per cent of these societies, masters and slaves were of the same ethnicity, with 21 per cent of different ethnicities and 4 per cent an ethnic mix of slaves. This suggests that slavery was not necessarily ethnic or racial in origin. Before 1500, moreover, Africans actually made up a minority of the Old World's slave population, which also suggests that the institution of slavery is not historically linked to those of African descent. The myriad and complex forces which combined in the late fifteenth century to produce the forced migration of millions of Africans to the European colonies of the New World, and the association between slavery and those born in Africa were not, therefore, predetermined or inevitable, although they did have their predecessors.