Broken glass pieces. Cullet can be melted in a glass blower's furnace (with or without the addition of glass batch) and can be used for casting in a kiln. Cullet is sometimes referred to as "nugget".
Cullet is used to describe both "new" glass produced in the cullet format as well as broken "used" glass being reclaimed in a recycling process.
The word cullet is a variant of the 19th century collet - glass left at the end of a blow pipe or punty.