The term ONE OF THE TWO MAIN BRANCHES OF ISLAM can be summarized by the crossword solution SHIA, which is an abbreviation for the Shia branch of Islam. Shia is one of the two main branches of Islam, the other being Sunni. Shia Muslims believe that Ali, the son-in-law of the Prophet Muhammad, was the rightful successor to the Islamic faith, while Sunni Muslims believe that Abu Bakr, a close companion of the Prophet Muhammad, was the rightful successor. The two branches have different interpretations of Islamic law and practice, and have been in conflict since the 7th century.