A ruling or opinion issued by an Islamic legal authority. It may be issued in response to a current political or moral controversy in which acceptable Muslim conduct has not yet been defined. 

In Sunni Islam, religious scholars called Muftis are qualified to issue fatwas. However as Sunni Muslims do not strictly adhere to one Mufti,  fatwas issued by them are considered non-binding. They are closer to a legal opinion more than a legal ruling unless incorporated into the judicial system of a nation. 

In Shi’ah Islam,  only a jurist at the highest level, known as a marja’, can issue a fatwa. Because Shi’ah Muslims perform taqlid (follow the guidance) of a chosen marja’, the fatwas issued are binding on their followers. Therefore, the fatwas of the Shi’ah Jurists are closer to legal rulings than opinions.