Don is short for the Latin word “dominus”, meaning lord. The use was originally reserved only to God, the ultimate Lord. Eventually it became used to designate the saints, and in the Basque language still retains that meaning (Donostia = San Sebastián), as well as in Italian (Don Bosco, Don Orione). Subsequently it was extended to the popes, kings and their close relatives, then to the bishops and prelates, and finally to titled nobility, who then made it hereditary.
The word "dominus" originally referred to power over men and land and has no link with noble origin. It is not a title but deferential treatment. However, it evolved later to highlight social prominence and got associated with the nobility.
In the second half of the XIV, the word "don" is used only by lay titled nobility only. For ecclesiastical use, "don" was closely related to their hierarchy, limited during the Middle Ages only to abbots, deans, bishops, archbishops, and other dignitaries.
In very rare cases was granted as a favor by kings only in exceptional cases (for example with Christopher Columbus, Hernán Cortés, Francisco Pizarro and Diego de Almagro).
Jews were not allowed to use the "don" neither as lords with power over land and men nor as titled nobility. Those allowed to use the "don" wrote it even in their signatures, as complement to their name, what attests to the high important assigned to it.
From the XVII century the word “don" was used in front of the name of bishops , titled nobility , gentlemen and children of titled nobility (even illegitimate children), but was never used for commoners .
In the mid-1700s the use extended to all nobility (with or without titles) and later in the American colonies the use applied to all Europeans with good social level.
In current times the use of "don" is widespread in Spain; however in many Spanish-speaking countries in Latin America it remains a sign of respect, reserved for older people or relevant in society.