Sunni Islam ( or ) is a denomination of Islam which holds that the Islamic prophet Muhammad's first  Caliph was his father-in-law Abu Bakr. Sunni Islam primarily contrasts with Shi'a Islam, which holds that Muhammad's son-in-law and cousin Ali ibn Abi Talib, not Abu Bakr, was his first caliph. Sunni Islam is by far the largest denomination of Islam. , Sunni Muslims constituted 87–90% of the world's Muslim population. Its adherents are referred to in Arabic as , "people of the tradition of Muhammad and the consensus of the Ummah" or (Arabic أهل السنة) for short. In English, its theological study or doctrine is called Sunnism, while adherents are known as Sunni Muslims, Sunnis, Sunnites and Ahlus Sunnah. Sunni Islam is the world's largest religious denomination, followed by Roman Catholicism. Sunni Islam is sometimes referred to as "orthodox Islam". The word "Sunni" comes from the term Sunnah (Arabic سنة), which refers to the sayings and actions of the Islamic prophet Muhammad as recorded in the hadith.