The Collaborative International Dictionary
Hygrophanous \Hy*groph"a*nous\, a. [Gr. ? wet + ? to show.] Having such a structure as to be diaphanous when moist, and opaque when dry.
Wiktionary
a. (context mycology English) Changing color as it dries out.
Wikipedia
Hygrophanous pileus of Psilocybe cyanescens
Hygrophanous pileus of Tubaria furfuracea
The adjective hygrophanous refers to the color change of mushroom tissue (especially the pileus surface) as it loses or absorbs water, which causes the pileipellis to become more transparent when wet and opaque when dry.
When identifying hygrophanous species, one needs to be careful when matching colors to photographs or descriptions, as color can change dramatically soon after picking.
Genera that are characterized by hygrophanous species include Agrocybe, Psathyrella, Psilocybe, Panaeolus, and Galerina.