Lit. ‘period of concealment’. Qadi Nu‘man (d. 974) uses the term dawr al-satr to refer to the period of around 150 years in which the Isma‘ili imams were hidden from public knowledge, and which ended with the appearance of ‘Abdallah (or ‘Ubaydallah) ‘al-Mahdi’, who in Nu‘man’s terminology started the period of disclosure (dawr al-kashf). According to Abu Ya‘qub al-Sijistani (d. after 971), dawr al-satr refers to the period when truth is concealed from the senses, that is, the period that started with Adam and which he expected to end upon the return of Muhammad b. Ismail as the Mahdi. Later, Nasir al-Din al-Tusi (d. 1274) speaks of periods of concealment that can take place when the imam’s true spiritual reality is not manifested, even if he is physically available.