The Aztecs () were a Mesoamerican culture that flourished in central Mexico in the post-classic period from 1300 to 1521. The Aztec people included different ethnic groups of central Mexico, particularly those groups who spoke the Nahuatl language and who dominated large parts of Mesoamerica from the 14th to the 16th centuries. Aztec culture was organized into city-states (altepetl), some of which joined to form alliances, political confederations, or empires. The Aztec Empire was a confederation of three city-states established in 1427: Tenochtitlan, city-state of the Mexica or Tenochca; Texcoco; and Tlacopan, previously part of the Tepanec empire, whose dominant power was Azcapotzalco. Although the term Aztecs is often narrowly restricted to the Mexica of Tenochtitlan, it is also broadly used to refer to Nahua polities or peoples of central Mexico in the prehispanic era, as well as the Spanish colonial era (1521–1821). The definitions of Aztec and Aztecs have long been the topic of scholarly discussion ever since German scientist Alexander von Humboldt established its common usage in the early 19th century.

Most ethnic groups of central Mexico in the post-classic period shared basic cultural traits of Mesoamerica, and so many of the traits that characterize Aztec culture cannot be said to be exclusive to the Aztecs. For the same reason, the notion of "Aztec civilization" is best understood as a particular horizon of a general Mesoamerican civilization. The culture of central Mexico includes maize cultivation, the social division between nobility (pipiltin) and commoners (macehualtin), a pantheon (featuring Tezcatlipoca, Tlaloc, and Quetzalcoatl), and the calendric system of a xiuhpohualli of 365 days intercalated with a tonalpohualli of 260 days. Particular to the Mexica of Tenochtitlan was the patron God Huitzilopochtli, twin pyramids, and the ceramic ware known as Aztec I to IV.

From the 13th century, the Valley of Mexico was the heart of dense population and the rise of city-states. The Mexica were late-comers to the Valley of Mexico, and founded the city-state of Tenochtitlan on unpromising islets in Lake Texcoco, later becoming the dominant power of the Aztec Triple Alliance or Aztec Empire. It was an empire that expanded its political hegemony far beyond the Valley of Mexico, conquering other city states throughout Mesoamerica in the late post-classic period. It originated in 1427 as an alliance between the city-states Tenochtitlan, Texcoco, and Tlacopan; these allied to defeat the Tepanec state of Azcapotzalco, which had previously dominated the Basin of Mexico. Soon Texcoco and Tlacopan were relegated to junior partnership in the alliance, with Tenochtitlan the dominant power. The empire extended its reach by a combination of trade and military conquest. It was never a true territorial empire controlling a territory by large military garrisons in conquered provinces, but rather dominated its client city-states primarily by installing friendly rulers in conquered territories, by constructing marriage alliances between the ruling dynasties, and by extending an imperial ideology to its client city-states. Client city-states paid taxes, not tribute to the Aztec emperor, the Huey Tlatoani, in an economic strategy limiting communication and trade between outlying polities, making them dependent on the imperial center for the acquisition of luxury goods. The political clout of the empire reached far south into Mesoamerica conquering polities as far south as Chiapas and Guatemala and spanning Mesoamerica from the Pacific to the Atlantic oceans.

The empire reached its maximal extent in 1519, just prior to the arrival of a small group of Spanish conquistadors led by Hernán Cortés. Cortés allied with city-states opposed to the Mexica, particularly the Nahuatl-speaking Tlaxcalteca as well as other central Mexican polities, including Texcoco, its former ally in the Triple Alliance. After the fall of Tenochtitlan on 13 August 1521 and the capture of the emperor Cuauhtémoc, the Spanish founded Mexico City on the ruins of Tenochtitlan. From there, they proceeded with the process of conquest and incorporation of Mesoamerican peoples into the Spanish Empire. With the destruction of the superstructure of the Aztec Empire in 1521, the Spanish used the city-states on which the Aztec Empire had been built, to rule the indigenous populations via their local nobles. Those nobles pledged loyalty to the Spanish crown and converted, at least nominally, to Christianity, and, in return, were recognized as nobles by the Spanish crown. Nobles acted as intermediaries to convey taxes and mobilize labor for their new overlords, facilitating the establishment of Spanish colonial rule.

Aztec culture and history is primarily known through archaeological evidence found in excavations such as that of the renowned Templo Mayor in Mexico City; from indigenous writings; from eyewitness accounts by Spanish conquistadors such as Cortés and Bernal Díaz del Castillo; and especially from 16th- and 17th-century descriptions of Aztec culture and history written by Spanish clergymen and literate Aztecs in the Spanish or Nahuatl language, such as the famous illustrated, bilingual (Spanish and Nahuatl), twelve-volume Florentine Codex created by the Franciscan friar Bernardino de Sahagún, in collaboration with indigenous Aztec informants. Important for knowledge of post-conquest Nahuas was the training of indigenous scribes to write alphabetic texts in Nahuatl, mainly for local purposes under Spanish colonial rule. At its height, Aztec culture had rich and complex philosophical, mythological, and religious traditions, as well as achieving remarkable architectural and artistic accomplishments.