Final answer:
The Arabic word for the period of concealment in Ismailism is 'satr,' referring to when Imams were hidden due to political or security reasons.
Step-by-step explanation:
The Arabic word for concealment that specifically refers to the period in which the Imams remained concealed or 'veiled' in Ismailism is satr. The term satr or sitr (meaning concealment or privacy) signals a historical period when the imamate was not publicly active or directly accessible to its followers. The Ismaili Imams during certain times for various reasons, such as political challenges or threats to their security, found it necessary to maintain secrecy about their whereabouts and even their identities, leading to what is referred to as the period of concealment or satr.