Aga Khan is the honorific title given to the imam of the Nizari Ismaili since the 19th century.
Ismailism is one of the biggest branches of the Shia Islam. After a split inside the Shia branch between the Mustalian and the Nizari, the Nizari were able to gain prominence, founding their own state that went from Persia to Syria, but which eventually collapsed. Despite this, the Nizari branch became recognized at the international level mainly in the 19th century and onwards, specially due to their spiritual leaders.
Hasan Ali Shah (1800 – 1881), descendent of the prophet Muhammad (through Muhammad’s daughter and son-in-law) and spiritual leader of the Ismaili, was the first to be granted the title of Aga Khan, by the Shah of Iran. However, after trying to revolt unsuccessfully against Iran, Aga Khan I fled to India. Many Nizaris had already established themselves in India, and with their close links to the British, they had been able to rise economical and politically in that country. Despite several quarrels with the Khojas, Aga Khan I and his successors rapidly won a position of respect in India’s Islam. Aga Khan III (1877 – 1957, also known as Sultan Sir Mohammed Shah), was the president of the All-India Muslim League, helped in the Indian constitutional reform, and eventually became president of the League of Nations in 1937. Although very marked by the Western culture, he remained very conscious of the need to fully establish the interests of the Muslim minority in India. His works concerning the Nizari Ismaili community are still praised today, since he created several institutions in favor of its social and economic development. He and his successor, Aga Khan IV (born in 1936, His Highness the Aga Khan, also know as Prince Shah Karim Al Hussaini Aga Khan), remain mostly known due to their philanthropic works. Indeed, the Aga Khan Development Network, founded by Aga Khan IV, includes a vast number of agencies, such as the Aga Khan Foundation, the Aga Khan Health Services or the Aga Khan University, and is one of the most important private institutions that “work[s] to improve the welfare and prospects of people in the developing world, particularly in Asia and Africa.”
Finally, the Aga Khan remain the Nizari Ismaili spiritual leaders, which is not negligible since this community is estimated to be composed of 15 million people. You can find out more about their work and their story here.