What is Satr?

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, 4th vol., p. 1304) by Edward William Lane, the word satr means to veil, conceal or hide a thing. The early Ismailis had employed the term satr with regards to those periods in their history when the Imams were hidden from the eyes of their followers. 

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."

Source