Ijtihad (Arabic: اجتهاد, ʼijtihād) is the making of a decision in Islamic law (sharia) by personal effort (jihad), independently of any school (madhhab) of jurisprudence (fiqh). as opposed to taqlid, following the decisions of a religious expert without necessarily examining the scriptural basis or reasoning of that decision.
To be valid and accepted it has to be rooted in the Qur'an and the hadith and it is required that no established doctrine rules the case. A mujtahid is an Islamic scholar who is competent to interpret sharia by ijtihad. Whereas Shi'a Muslim outright reject ijtihad and do not imitate a mujtahid who practice ijtihad.
Read more about Ijtihad: Etymology and Definition, History, Qualifications of A Mujtahid, Types of Mujtahideen, Modern Application