A sultan is a secular ruler, usually in an Islamic country. The name came into use as the nation of Islam far extended its territories and relied on secure governance in far away provinces and countries. The sultan, as ruler, was initially inferior to the ruling caliph, but generally governed with almost absolute authority in a particular area.
Unlike the caliph, the title sultan did not denote that the sultan was head of the Islamic religion. Caliphs were first chosen as the successors to Muhammad, and not only headed the Islamic people but the religion of Islam. This would quickly become a point of contention between groups of Islam, when the Shi’a groups declared the caliph had to be a blood descendent of Muhammad. Disagreement when the Umayyad family took the caliphate lead to the Shi’a/Sunni schism.
Using the term sultan, instead of caliph, was one way to separate the political from the religious. It was an inoffensive term that did not evoke old battle lines that had been drawn around the issue of the head of the state also leading the religion. Though the sultan should be strongly moral and an upright person, his interest in guiding the religious thoughts of the people was minimal, and he normally deferred to the religious leaders in his country.
The great sultans arose during the Ottoman Empire. Rulers of this Turkish land were normally designated as sultan. Other rulers in countries, who did not wish to challenge the authority of the caliphate, like those in Egypt, styled themselves sultans. In fact, during the apex of Ottoman Empire control, caliphs actually specifically used the term sultan to describe rulers of the country.
Today there are sultans still, with greater or lesser power depending on their area of rule. The term sultan is a common one for rulers in Malaysia, Brunei, and Oman. It remains a title of authority for primarily Muslim leaders, and is not commonly used outside of the Muslim world. Many leaders of countries, who would formerly have been called sultans, now refer to themselves as king.
An interesting point in the use of the word sultan is its earliest usage. When first used, sultan often meant wife of a ruler, not the ruler himself. So the term, though it translates as authority or strength, usually meant lesser authority or strength. The term sultana, which has been used to describe the wife of a sultan, is a great misunderstanding of what sultan means, and primarily a Western corruption. From a feminist standpoint, the fascinating aspect of designating ruler’s wives as sultans suggests the greater power women held in the early Islamic community. To be considered strong or an authority was truly the province of women married to rulers. In a way, they led too, though they did so under the authority of their husbands and subject to their husband's rule.