Breathability
Goretex was the first fabric, which could hold out water and breathe vapor at the same time. The principle is having a membrane, which is waterproof, but with small holes punched in it—small enough for steam to evaporate through them, but too small for fluid water to enter. A simple principle, but probably not that simple to turn into a real-life product. The price of Goretex reflected that—and to some extent still does.
Since the introduction of Goretex a lot of similar products have seen the light of day, and breathable membranes can be found everywhere today and at fair prices.

Vapours escape

Vapours trapped