The name for the window glass used in the early 1800s. Back then, windows were formed by blowing a large bubble of glass, then spinning it until it was flat. This left a sheet of glass with a "crown" bump in the center. This caused a wavy refractive distortion when looking through the glass.

Today, window glass is made flat by letting it spread itself out under it's own weight while floating on top of a liquid surface, so that there are minimal distortions.