ijtihad
Ijtihad means “exertion”: the effort of a jurist (mujtahid) to derive a rule from the sources where the Quran and Sunna give no direct answer. It works through methods such as qiyas (analogy), istihsan (juristic preference) and istislah (public interest). A person who follows a school's established rulings without independent reasoning practises taqlid (imitation).
Many Sunni writers held that “the gate of ijtihad” was closed around the tenth century, after the four schools were settled, though this view is disputed. Shia law has always allowed living mujtahids to exercise it. Reformers in South Asia, including Iqbal, argued for reopening it; Indian courts, however, apply the established rules of the schools and do not themselves exercise ijtihad.
Example
Modern writers who argue that triple talaq should not be recognised often rely on ijtihad to revisit classical Hanafi rulings.