An iwan is a rectangular hall, vaulted and walled on three sides, that opens completely on its fourth side onto a courtyard through a tall arched or trabeated frame. In mosque architecture the iwan provides a monumental, shaded threshold space that mediates between the open sahn and any enclosed hall behind it, and its scale, often rising well above the height of the surrounding walls, makes it the most visually commanding element of the facades that frame the courtyard.
The form is defined by its openness rather than its size: a small madrasa cell and a vast royal reception hall can both be called an iwan if they share the three-walled, one-open-sided arrangement and the framing arch. In the four-iwan mosque and madrasa plan that became standard across Iran, Central Asia and much of the wider Persianate world from the Seljuk period onward, four iwans face one another across a shared courtyard, one on each side, with the iwan on the qibla side typically the largest and serving as the principal covered prayer space, often opening directly onto or containing the mihrab. Iwans also scale down for smaller purposes without losing the defining arrangement: a madrasa built around the four-iwan plan gives each of its four iwans over to a different school of Islamic law, or reserves the smaller two for teaching cells and student lodging while the largest pair frame the courtyard's main axis, so the same three-walled, one-open-sided form serves devotional, educational and residential roles within a single complex.