A qadi is a judge appointed by the ruler to decide cases under Islamic law, and his defining feature is that his judgment is binding and carried out, which sets him apart from the scholar who only gives opinions.
The office of judgment
The qadi is appointed by the political authority to hear and decide legal cases according to the sacred law, and his rulings are enforced by the power of the state, so he exercises a real judicial office with coercive effect. This is the clean contrast with the mufti: the mufti gives a reasoned opinion, a fatwa, that binds no one and can be sought again elsewhere, while the qadi decides a case between parties and his decision stands and is executed. The qadi applies the law to the facts before him and delivers a verdict.
Appointment and independence
Because the qadi is appointed by the ruler, the office is bound up with political authority in a way the scholarly offices are not, and the relationship between the judge duty to the law and his dependence on the power that appoints and can dismiss him has been a standing tension in Islamic legal history, the ideal of the upright judge who will not bend to the ruler being celebrated precisely because the pressure was real. The qadi is trained in a school of law and appointed to the bench, and his court has historically handled the whole range of legal business. The office corresponds to the judge in any legal system, its distinctive feature within Islam being its application of the sacred law and its position between that law and the state that enforces it.