When Americans refer to the Western and Eastern worlds, they generally mean the Christian and Muslim worlds. Originally, the West was the Catholic Western Roman Empire, with its capital in Rome, while the East was the Eastern Orthodox Roman Empire, centred around Byzantium. Much of the latter was conquered by Muslims, so the East was seen as non-Christian realms. 

After the spread of Islam, the African side of the Mediterranean became Muslim, and was seen as part of the Eastern world, where the northern side that stayed Christian was seen a Western. Thus, notions of East and West depend on who is writing the history. In colonial times, it was the Christian historians of Europe.