A set of principles whereby rules of Islamic jurisprudence (fiqh) are derived from specific evidences in the main shari’a sources (the Quran and Sunnah) and secondary shari’a sources (ijma’a and qiyas). Usul (singular. asl) refer to a body of principles which jurists use in law derivation or deduction. Therefore, the purpose of usul al-fiqh is to deduce the rules of fiqh from the evidence and clues found in the sources of shari’a. Usul al-fiqh explains how Islamic law should be worked out from available sources and how it can be classified and put into application.

Usul al-fiqh (in Arabic script: أصول الفقه) stands for roots of the law (shari’a) or theoretical jurisprudence or Islamic legal theory.