The word sultan describes the ruler of an Islamic country; his realm is known as a sultanate. The English language word sultan is derived from the Arabic term sultan, meaning "power" or "rule," which in turn comes from the Aramaic word shelet, meaning "to have power."
An interesting feature of the word sultan is that it implies absolute political authority, but not religious authority. Whereas many other Asian and European rulers were heads of both the faith and of the state, a sultan owed religious allegience to the caliph (at least in theory).
The term sultan is strongly associated with the Seljuk (1016 - 1153 CE) and Ottoman (1299 - 1923 CE) Empires, based in what is now Turkey. In fact, the pre-Seljuk Turkish ruler Yavuz Ozgur (r. 998 - 1030 CE) was the first to be named sultan.
At various times in history, the leaders of Islamic states from Morocco to the western borders of China, and from Nigeria to Indonesia, have used this title. Today, sultans head several modern Islamic nations, including Brunei and Oman, as well as smaller states within the federation of Malaysia and the southern Philippines.