noun
inner window; secondary window
Refers to an additional window installed on the interior side of an existing window, often for thermal insulation, soundproofing, or condensation prevention. Contrasts with 外窓 (そとまど, outer window).
See also: 外窓
冬の寒さ対策に内窓を設置した。
I installed an inner window as a measure against the winter cold.
内窓を閉めると外の騒音がかなり減る。
Closing the inner window reduces outside noise considerably.