Original: Tenochtitlan (sometimes also known as Mexico Tenochtitlan or Tenochtitlan Mexico) was a Nahua altepetl (city-state) located on an island in Lake Texcoco, in the Valley of Mexico. Founded in 1325, it became the seat of the growing Aztec Empire in the 15th century, until captured by the Spanish in 1521.
Simplified: Tenochtitlan was the massive capital city of the Aztec empire, built around 1325.