Part of speech: Adjective
so thin as to transmit light
Example: a hat with a diaphanous veil; filmy wings of a moth; gauzy clouds of dandelion down; gossamer cobwebs; sheer silk stockings; transparent chiffon; vaporous silks
The word "diaphanous" comes from Ancient Greek διαφανής, which comes from Latin diaphanus.
Ancient Greek had multiple words for concepts English expresses with just one, showing different cultural priorities.