An iwan is a "large niche, usually barrel-vaulted, open on one side."1 Humayun's Tomb has a pishtaq at the center of all four of its main facades, and an iwan is located in the northern, eastern, and western facades' pishtaqs.
1. D. Fairchild Ruggles, Islamic Gardens and Landscapes (Philadelphia: The University of Pennsylvania Press, 2008), 225.
Brand, Michael. "Orthodoxy, Innovation, and Revival: Considerations of the Past Imperial Mughal Tomb Architecture." In Muqarnas 10: An Annual on Islamic Art and Architecture, edited by Margaret B. Sevcenko, 323-34. Leiden: E.J. Brill, 1993.
Golombek, Lisa, and Ebba Koch. "The Mughals, Uzbeks, and the Timurid Legacy." In From the Mongols to Modernism, edited by Finbarr Barry Flood and Gülru Necipoglu, 811-45. Vol. 2 of A Companion to Islamic Art and Architecture. Oxford: Wiley Blackwell, 2017.
Hillenbrand, Robert. "Mughal Architecture Explored." South Asian Studies 12, (1996): 105-23.
Petersen, Andrew. Dictionary of Islamic Architecture. London; New York: Routledge, 1996.