tercio

A tercio (, Spanish for '[a] third') was a military administrative unit of the Spanish Army during the reign of the Catholic Monarchs of Spain and Habsburg Spain in the early modern period. They were the elite military units of the Spanish monarchy and essential pieces of the powerful land forces of the Spanish Empire, sometimes also fighting along with the navy. These forces were among the most dominant in the European battlefields for more than a century and a half.

The Spanish tercios were some of the finest and most influential professional infantry forces in the world due to the effectiveness of their battlefield formations, and were the crucial step in the formation of modern European armies, made up of professional volunteers, instead of levies raised for a campaign or hired mercenaries typically used by other European countries of the time. The internal administrative organization of the tercios and their battlefield formations and tactics grew out of the innovations of Gonzalo Fernández de Córdoba during the conquest of Granada and the Italian Wars in the 1490s and 1500s, being among the first to effectively mix pikes and firearms (arquebuses).