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.