From Arabic; derived from the root ja ha da, ‘to make an effort’. Mujtahid literally means one who strives for a higher position, usually in scholarship and learning. In Muslim law, the term refers to a person who is equipped with the knowledge and authority to perform ijtihad. The term was extensively used from the middle of the 2nd AH/8th CE century until the 4th AH/10th CE century when the four Sunni schools of law were constituted. Amongst Sunni Muslims, the term mujtahid is restricted to legal scholars of the medieval period. Among the Shi‘a, the Imam holds ultimate authority in matters of Muslim law. For the Twelver Shi‘is, following the concealment of their twelfth Imam, authority over the law was delegated to their ‘ulama’ who excercise ijtihad in formulating their own religious and judicial principles, based on the teachings of the Holy Qur’an and the Sunna of the Prophet as attested or verified by the Imams.