Tenochtitlan was a large city-state of the Mexica people that was founded in 1325 and then became the capital of the Aztec Empire until its conquest by the Spanish in 1521. When it was built, it was constructed similarly to Venice, with small islands on Lake Texcoco. Lake Texcoco then dried out and Mexico City was built where Tenochtitlan once stood, where the ruins of it are now the historical center of Mexico City.
Tenochtitlan's full name in Nahuatl is Mēxihco Tenōchtitlan. The origin of its name are uncertain, but Tenochtitlan is thought to mean "prickly pear growing in the rocks". Alternatively, it is thought it was named after the Mexica's founding leader, Tenoch.
The correct pronunciation of Tenochtitlan is Teh-nosh-TEE-tlahn. The "ch" is pronounced as an English "sh" sound. It is important to stress the third syllable, where the "i" is pronounced with a long "e", like when you find a double "e" in English, as in the word "see". The final syllable, -tlahn, contains an "a" that is pronounced as an "ah" sound.