crown glass

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A soda-lime optical glass that is exceptionally hard and clear, with low refraction and low dispersion.
• noun A form of window glass made by whirling a glass bubble to make a flat circular disk with a lump left in the center by the glass blower's rod.

from The Century Dictionary.

• noun A good quality of common blown window-glass.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a glass blown into a globe which is later flattened and spun to form a disk
• noun optical glass of low dispersion and low refractive index