'abû muhammadi calî bni ussayn (full name) zaynu l-cabidîn (honorary title) 'as-sajjâd (honorary title) 'az-zakiyy (honorary title)
(Madina 658 - 712 or 713) The fourth Imam of Shi'i Islam (680- 712 or 713).
Due to his weak health and inability to fight he was the only son of Husayn to survive the massacre at Karbala. He was taken as prisoner to Damascus but was freed by Yazid and allowed to return to Madina. He spent his life in seclusion, weeping over the martyrs at Karbala, for which he was named 'as-Sajjad (the prostrator). He did not involve himself in the politics of his time and was widely well regarded for his piousness. Other honorary titles of him was the most commonly used, Zaynu l-Abidin (the ornament of the worshippers), and 'az-Zaki (the pure).
He was succeeded by Muhammadu l-Baqir in the Twelver and Ismalili traditions and by Zayd in the Zaydi tradition.