Aztlán (from Nahuatl languages: Aztlán, Nahuatl pronunciation: [ˈast͡ɬãːn̥] ^ⓘ) is the ancestral home of the Aztec peoples. The word "Aztec" was derived from the Nahuatl aztecah, meaning "people from Aztlán." Aztlán is mentioned in several ethnohistorical sources dating from the colonial period, and while each cites varying lists of the different tribal groups who participated in the migration from Aztlán to central Mexico, the Mexica who later founded Mexico-Tenochtitlan are mentioned in all of the accounts.

Historians have speculated about the possible location of Aztlán and tend to place it either in northwestern Mexico or the Southwestern United States, although whether Aztlán represents a real location or a mythological one is a matter of debate.

Nahuatl histories relate that seven tribes lived in Chicomoztoc, or "the Place of the Seven Caves". Each cave represented a different Nahua group: the Xochimilca, Tlahuica, Acolhua, Tlaxcalteca, Tepaneca, Chalca, and Mexica. Along with these people, the Olmec-Xicalanca and Xaltocamecas are also said to come from Aztlán. Because of their common linguistic origin, those groups are termed collectively "Nahualteca" (Nahua people). These tribes subsequently left the caves and settled "near" Aztlán.

The various descriptions of Aztlán apparently contradict each other. While some legends describe Aztlán as a paradise, the Codex Aubin says that the Aztecs were subject to a tyrannical elite named the Azteca Chicomoztoca. Guided by their priest, the Aztec tribe fled. On the road, their god Huitzilopochtli forbade them to call themselves Azteca, telling them that they should be known as Mexica. Scholars of the 19th century—in particular Alexander von Humboldt and William H. Prescott—translated the word Azteca, as is shown in the Aubin Codex, to Aztec.

The southward migration is estimated to have begun on May 24, 1064 CE, based on the dates of the supernova Crab Nebula from May to July 1054. Each of the seven groups is credited with founding a different major city-state in Central Mexico.

A 2004 translation of the Anales de Tlatelolco gives the only known date related to the exit from Aztlán; day-sign "4 Cuauhtli" (Four Eagle) of the year "1 Tecpatl" (Knife) or 1064–1065, and correlated to January 4, 1065.

Cristobal del Castillo mentions in his book "Fragmentos de la Obra General Sobre Historia de los Mexicanos", that the lake around the Aztlán island was called Metztliapan or "Lake of the Moon." Another version reads:

One day a man heard a bird calling to him, saying, "Go now, go now." When the man told the chief about the bird, the chief was relieved. He had known his people must find a new land, their own land, but had waited for a sign. So the people gathered and began a long march. They followed an idol of Huitzilopochtli that the priests carried. As they went, Huitzilopochtli spoke through the priests and prepared the people for the greatness of their empire to come. He explained that they should travel until they came to a large lake; there, they should look for another sign—an eagle in a cactus.