A jurist's exertion of his utmost effort inquiring into the sources of law in an effort to arrive at a ruling on a situation for which no agreed-upon ruling yet exists.
It is often said that the "gates of ijtihad" were closed in the medieval period, as jurists relied more and more on previous rulings and engaged less and less in original interpretations of the sources. In fact some amount of ijtihad has always been necessary for extending Islamic law to cover new situations, but traditionalists now consider that very few scholars are qualified to perform ijtihad. Many modernists and islamists, however, regard the reform of Islam through ijtihad as the duty of all muslims.