Ali Zayn al Abidin (658 - 713) was the fourth Shia Imam. He was the son of Husayn ibn Ali and the great-grandson of Muhammad. His given birth name was Ali ibn Husayn. The title of Zayn al Abidin means ornament of the pious.
Ali Zayn al Abidin was born in Medina in 658 to Husayn and his wife Shahr Banu who was the daughter of the last Sassanid emperor, Yazdegerd III. His mother would not live long after his birth. He was about two years old when his grandfather, Ali ibn Abu Talib was killed. When his father Husayn set out for Iraq, Ali was not able to go with him due to illness. Before his death, Husayn left orders to pass on his position to his eldest son should he die. At the Battle of Karbela, Husayn and most of his family were killed. Ali survived because he was too sick to fight, and was forced to stay in bed. Afterwards, he was taken prisoner by the Umayyad forces and transported to Damascus where he was made a prisoner of Yazid I. Eventually, he was freed, and he returned to Medina where he lived in general seculsion, although he did engage in preaching and teaching of Islam.
Ali Zayn al Abidin would die in 713, some claim that he was poisoned. He is buried in the Jannat ul-Baqi cemetary in Medina.
The issue of who suceeded him as Imam led to a split within Shiism. While the Imami Shia believe that it was Muhammad al Baqir, the Zaidis believe it was Ali Zayn's other son, Zaid ibn Ali.