(c. 763—809) fifth Abbasid caliph of Baghdad 786–809
Abbasid caliph whose exploits are the subject of The Thousand and One Nights. The Abbasid caliphate reached its apogee under his rule, which was characterized by relative peace, prosperity, and unity of the Islamic community. Corresponded with and sent a diplomatic mission to the Holy Roman Emperor, Charlemagne, opening a period of technological and economic exchange with the West. Established a hostel for Christian pilgrims in Jerusalem. Set up the first hospital and observatory in Baghdad, which served as models for later rulers.