The Aztecs ( AZ-teks) were a Mesoamerican civilization 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, the capital city of the Mexica or Tenochca, Tetzcoco, 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 essential cultural traits of Mesoamerica. So many of the characteristics 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.