Nahuatl-speaking people who in the 15th and early 16th centuries ruled a large empire in what is now central and southern Mexico. The Aztec are so called from Aztlán ("White Land"), an allusion to their origins, probably in northern Mexico. They were also called the Tenochca, from an eponymous ancestor, Tenoch, and the Mexica, probably from Metzliapán ("Moon Lake"), the mystical name for Lake Texcoco. From "Tenochca" was derived the name of their great city, Tenochtitlán; and from "Mexica" came the name for the city that superseded the Aztec capital and for the surrounding valley, which was applied later to the whole Mexican nation. The Aztec referred to themselves as Culhua-Mexica, to link themselves with Colhuacán, the center of the most civilized people of the Valley of Mexico.

The origin of the Aztec people is uncertain, but elements of their own tradition suggest that they were a tribe of hunters and gatherers on the northern Mexican plateau before their appearance in Meso-America in perhaps the 12th century; Aztlán, however, may be legendary. It is possible that their migration southward was part of a general movement of peoples that followed, or perhaps helped trigger, the collapse of the Toltec civilization. They settled on islands in Lake Texcoco and in AD 1325 founded Tenochtitlán, which remained their chief center. The basis of the Aztec's success in creating a great state and ultimately an empire was their remarkable system of agriculture, which featured intensive cultivation of all available land, as well as elaborate systems of irrigation and reclamation of swampland. The high productivity gained by these methods made for a rich and populous state.

Under the ruler Itzcóatl (1428-40), Tenochtitlán formed alliances with the neighboring states of Texcoco and Tlacopan and became the dominant power in central Mexico. Later, by commerce and conquest, Tenochtitlán came to rule an empire of 400 to 500 small states, comprising by 1519 some 5,000,000 to 6,000,000 people spread over 80,000 square miles (207,200 square km). At its height, Tenochtitlán itself covered more than 5 square miles (13 square km) and had upwards of 140,000 inhabitants, making it the most densely populated settlement ever achieved by a Meso-American civilization. The Aztec state was a despotism in which the military arm played a dominant role. Valor in war was, in fact, the surest path to advancement in Aztec society, which was caste- and class-divided but nonetheless vertically fluid. The priestly and bureaucratic classes were involved in the administration of the empire, while at the bottom of society were classes of serfs, indentured servants, and outright slaves.

Aztec religion was syncretistic, absorbing elements from many other Meso-American cultures. At base, it shared many of the cosmological beliefs of earlier peoples, notably the Maya, such as that the present Earth was the last in a series of creations and that it occupied a position between systems of 13 heavens and 9 underworlds. Prominent in the Aztec pantheon were Huitzilopochtli, god of war; Tonatiuh, god of the sun; Tlaloc, god of rain; and Quetzalcóatl, the Feathered Serpent, who was part deity and part culture hero. Human sacrifice, particularly by offering a victim's heart to the sun god, was commonly practiced, as was bloodletting. Closely entwined with Aztec religion was the calendar, on which was based the elaborate round of rituals and ceremonies that occupied the priests. The Aztec calendar was the one common to much of Meso-America, and it comprised a solar year of 365 days and a sacred year of 260 days; the two yearly cycles running in parallel produced a larger cycle of 52 years.

The Aztec empire was still expanding, and its society still evolving, when its progress was halted in 1519 by the appearance of Spanish explorers. The ninth emperor, Montezuma II (reigned 1502-20), was taken prisoner by Hernán Cortés and died in custody. His successors, Cuitláhuac and Cuauhtémoc, were unable to stave off Cortés and his forces, and, with the Spanish capture of Tenochtitlán in 1521, the Aztec empire came to an end.