An empire is a political unit which exercises control over territory of great extent or a number of territories or peoples through formal annexations or various forms of informal domination.^[1]

Well-known empires include the Persian Empire, founded by Cyrus the Great,^[2] the Roman Empire, Arab Muslim empires such as the Rashidun Caliphate and the Umayyad Caliphate, Berber empires such as the Almohads and the Almoravids, the Russian Empire, the Greek Macedonian Empire, the Holy Roman Empire and the British Empire. The oldest empire was the Akkadian Empire, whose founder was Sargon of Akkad.

Empires that are known for conquering much territory with speed include the Greek Macedonian Empire (during the time of Alexander the Great) and the Arab Empire. According to Scottish historian James Buchan: "In speed and extent, the first Arab conquests were matched only by those of Alexander the Great, and they were more lasting."^[3]

Sea empires grew mostly by sailing to distant places and conquering them. Mediterranean sea empires were the Phoenician states of Tyre, Sidon and Carthage; the Italian maritime republics of Venice and Genoa. Others were the Chola dynasty of Tamil Nadu in India; the Omani Empire of Arabia; and the Austronesian empires of Srivijaya and Majapahit in Maritime Southeast Asia. The Aztec Empire and the Mongol Empire were land empires.

Before Christopher Columbus, there were several empires in America including the Aztec Empire, and the Inca Empire. The Spanish and the Portuguese were the first Europeans to discover America. Later came the English, later to be called the British Empire which controlled most of North America, and the Spanish Empire controlled most of South America and Latin America.

The newest created empire was The Soviet Union created in 1922, which included Russia, Kazakhstan, Kyrgyzstan, Tajikistan, Uzbekistan, Turkmenistan, Latvia, Estonia, Lithuania, Belarus, Ukraine, Moldova, Azerbaijan, Armenia and Georgia.

Today, the only monarch to use the title "emperor" is the Emperor of Japan although his power is mostly ceremonial and the de facto head of government is the Prime Minister of Japan.