Mother glass is the name given to the large, fabricated glass substrate that is cut into smaller individual pieces for plasma and LCD displays.
Mother glass is the largest possible size of glass that a factory can manufacture that will not break under its own weight. The larger the mother glass is, the more screens that can be cut from it, which saves time and money in the manufacturing line.
Techopedia explains Mother Glass
Mother glass gets its name because it, in a sense, gives birth to many smaller flat-panel TV screens, usually plasma screens. Most of what makes a screen, the pixels and what connects them, are "grown" on the mother glass through layering materials and using chemical processes to shape them.
The size of the mother glass gets progressively larger per generation of technology used.