The process of using one’s ‘Aql (reason) and learned Islamic knowledge to answer questions on Islamic law or conduct.

In Shi’ah Islam, the practice of ijtihād is reserved for jurists who have achieved the status of Mujtahid. Those who are not  Mujtahid practice the opposite of ijtihād, taqlid, i.e. following the guidance of a Mujtahid and their rulings. 

A Mujtahid uses several tools in the process of ijtihād: the Qurʾān, Sunnah, hadith of the  Prophet (ṣ) and the Ahl al-Bayt, Ijma (scholarly consensus), and their ‘Aql (reason).