noun
openable panel in a door; door panel that can be opened
Architectural term for a small hinged or sliding panel set within a larger door, allowing ventilation or a view without opening the whole door.
古い日本家屋の玄関には、無双窓が付いていることがある。
The entrance of old Japanese houses sometimes has an openable panel in the door.
The exact derivation is uncertain; the kanji 無双 (peerless) may refer to the unique dual-function design of the panel.