Why the Aztec's were called Aztec's?

The Aztecs original name is "Mexica", europeans called them Aztecs because of Aztlan, the area basically western u.s.(California) It's not the proper name to call them unless they we"re still in Aztlan, Mexica are what they are. You pronounce it like Mexico but the x makes a sh sound. ME-SHE-KA