Allowing light to pass through diffusely:
— Semitransparent curtains at the windows.

synoniem: translucent.

Roget 427: semitransparent, translucent, semipellucid^†, semidiaphanous^†, semiopacous^†, semiopaque; opalescent, opaline^†; pearly, milky; frosted, nacreous.