"The word satr (pl. satur) is derived from astar, meaning hide, cover or shield. As it is said, masatra (he concealed enmity), or tastir (to hold within a curtain). According to Arabic-English Lexicon (New York, 1872, 4:1304) by Edward William Lane, the word satr means to veil, conceal or hide a thing. The Ismailis had employed the term dawr-i satr (period of concealment) with regards to those periods in their history when the Imams were hidden from the eyes of their followers when the animosity of their enemies reached to its extreme. On that critical moment, the hujjats represented the Imams in the community. The hujjat was a living proof, acting as the custodian until the time of the Imam's reappearance. In contrast, the period following the concealment is known as dawr-i kashf (unveiling period) or the dawr-i zuhur (period of manifestation), when the Imams publicly made their appearance.
With the death of Imam Ismail (d. 158/775) and Imam Muhammad (d. 197/813), the gravity of brutal persecutions of the Abbasids had considerably increased. The Abbasids left no chance to grind the Ismailis under the millstone of cruelty. The Ismaili Imams were impelled to thicken their hiding, therefore, the first dawr-i satr came into force from 197/813 to 268/882, wherein the Imams were known as al-A'immatu'l masturin i.e., the concealed Imams. Idris Imaduddin (d. 872/1468) writes in Zahru'l-ma'ani (p. 59) that, "He (Wafi Ahmad) was the first of the three concealed Imams by the order of God and His inspiration." Hamiduddin Kirmani (d. 412/1021) also admits in his ar-Risalat al-Wai'za (comp. 408/1017) that, "Muhammad bin Ismail became qaim, and after him, the concealed Imams (aima'i masturin) succeeded to the Imamate, who remained hidden on account of the persecution of the tyrants, and these were three Imams, viz., Abdullah, Ahmad and Hussain." Hatim bin Imran bin Zuhra (d. 498/1104) writes in al-Usul wa'l Ahakam that, "When Muhammad bin Ismail died, his authority passed to his son, Abdullah bin Muhammad, the hidden one, who was the first to hide himself from his contemporary adversaries." According to Hasan bin Nuh Broachi (d. 939/1533) in Kitab al-Azhar (comp. 931/1525) that, "The three hidden Imams were Abdullah bin Muhammad, Ahmad bin Abdullah, surnamed at-Taqi and Hussain bin Ahmad." The fact that the dawr-i satr virtually came into force in the time of Imam Wafi Ahmad has been also asserted by the modern scholars, such as W. Ivanow, Dr. Sami Nassib Makarem, Sir Johj Glubb, Husayn F. al-Hamdani, etc.
Shahrastani (1076-1153) writes in Kitab al-milal wa'l nihal (p. 164) that, "Then begins the era of the hidden Imams, who went about secretly but sent out emissaries, who appeared openly on their behalf. They hold that the world can never be without an Imam who is alive and a qaim, either visible and manifest, or hidden and concealed. When the Imam is manifest it is possible for his hujjat (proof) to be hidden, but if the Imam is hidden it is necessary for his hujjat and emissaries to be manifest."
On account of the strictness of Imam's concealment, when his hujjats were accepting on his behalf the oath of allegiance from neophytes, they taught them that they should obey the Lord of the Time (Sahib al-Asr or Waliyul Asr) without pronouncing the name of the Imam. This practice was in use among the neophytes through the whole period of the concealment of the Imams.
And here we cannot but call attention to a fact that the doctrine of ghayba among the Twelvers should not be confounded with that of the concept of satr among the Ismailis. Seyyed Hossain Nasr writes in this context in his Ideals and Realities of Islam (London, 1966, p. 159) that, "The idea of being hidden (mastur) must no, however, be confused with the occultation (ghayba) of the twelfth Imam (of the Twelvers). The first implies simply being hidden from the eyes of the crowd and from public notice, while the second means disappearance from the physical world."
The second dawr-i satr in the Ismaili history took place between 490/1097 and 559/1164, wherein three Imams lived in concealment, viz. al-Hadi, al-Muhtadi and al-Kahir. During the period of satr, the Ismaili hujjats governed the Nizari state in Alamut, viz. Hasan bin Sabbah, Kiya Buzrug Ummid and Muhammad bin Kiya. The qiyamat-i qubra was a famous occasion commemorated in Alamut on 17th Ramzan, 559/August 8, 1164, when Imam Hasan II came out publicly upon the termination of dawr-i satr, which was replaced by dawr-i kashaf.
According to Cambridge History of Iran (London, 1968, 5:474), "The term satr had originally referred to those periods when the whereabouts of the Imam was unknown to the world at large, or even, at times, to the faithful, as had been the case among Ismailis before the rise of the Fatimids and again after the death of Nizar."