Asiento - also known as the asiento de negros (Spanish: "Negroes' contract"), the asiento was used from the early 16th to the mid-18th centuries C.E. and was an agreement between the Spanish crown and a private person or another sovereign power by which the latter was granted a monopoly in supplying African slaves for the Spanish colonies in the Americas.
The contractor (asentista) agreed to pay a certain amount of money to the crown for the monopoly and to deliver a stipulated number of male and female slaves for sale in the New World markets. Until the 18th century individual Spaniards, as well as Portugal, France, and Great Britain, entered into such contracts. In spite of heavy taxation, government interference, and unsettled trade conditions, all of which greatly curtailed the profitability of asientos, foreign powers sought the contracts because they provided the chance, however slim, to amass some of the gold and silver bullion produced by the slave trade. Source: Peopling North America: Population Movements & Migration