Al-Ghayba (Persian ghaybat), literally "the hiding," is sometimes translated as "the Occultation." While a number of early Shi˓ite theological groupings proposed that their imam had gone into "hiding," it was the Twelver Shi˓a, the only such group to survive into the classical period in any significant numbers, who fully developed the doctrine. Proclaiming that one's imam had gone into hiding had a number of advantages for persecuted Shi˓ite groups. First, it reduced their explicit challenge to the established political order. A hidden imam is (potentially) less disruptive than a manifest imam, thereby reducing political tension with the ruling Sunni authorities. Second, if this imam is predicted to return at some point, the community can be charged with merely waiting (intizar) for his return, rather than actively agitating against the governing political powers. Third, while the Shi˓a had divided into various groups, based around the charisma of particular would-be imams, a hidden imam could act as a unifying factor, as personality conflicts between imams were avoided.
The majority of Shi˓a settled upon both an individual and a point in time when the imam went into hiding. The individual was Muhammad, son of Hasan al-˓Askari (a descendent of Imam ˓Ali and proclaimed as the eleventh imam), and the time was 868 c.e. According to Shi˓ite reports, Hasan died when Muhammad was only six. Muhammad, also referred to as the Mahdi, went into hiding in order to avoid persecution from the Abbasid rulers. At first, he continued to communicate with his Shi˓a through intermediaries. These four intermediaries (known as "gates" or "ambassadors") passed on the orders of the hidden imam. After sixty-nine years (in 941), when the fourth agent was close to death, the imam announced that from that point on there were no further agents. While the imam was not leaving the world, he would remain in hiding until God decreed an appropriate time for his return. This ended the lesser occultation (alghayba al-sughra), and the greater occultation (al-ghayba alkubra) began. The Shi˓a are still awaiting the return of the imam, known as the Mahdi.
This doctrine appears to have taken some time to reach its final formulation, and later it was subjected to extensive theological justification. For example, in the eleventh century Muhammad b. al-Hasan al-Tusi, in his Kitab al-Ghayba, outlined both textual and rational justifications that later became common in Shi˓ite texts of theology. He argued that God would not leave his community without a guide—for to do so would entail his neglect of the Shi˓a and hence his injustice. There must, then, be an imam present in the world who acts as God's guide, and this imam must be sinless. Because there is no manifest imam who is both sinless and recognizable as the emissary of God, the imam must, therefore, be in hiding.
The doctrine of the ghayba also has a number of legal consequences. For example, the zakat and khums taxes, collected by the imam, become problematic. Eventually, Shi˓ite jurists avoided these duties being lapsed by proposing the doctrine of niyaba (deputyship) of the scholars to carry out these functions.
Arjomand, Said Amir. "The Consolation of Theology: The Shi˓ite Doctrine of Occultation and the Transition from Chiliasm to Law." Journal of Religion 76, no. 4 (1996): 548–571.
Kohlberg, Etan. "From Imamiyya to Ithna-˓Ashariyya." In Belief and Law in Imami Shi˓ism. Edited by E. Kohlberg. Hampshire, U.K.: Variorum, 1991.