Glass that is made so as to be particularly hard; (in later use) specifically borosilicate glass, which is especially resistant to the effects of heat and chemical action.
Origin
Late 16th century. From hard + glass.
Definition of hard glass in US English:
hard glass
noun
Glass that is made so as to be particularly hard; (in later use) specifically borosilicate glass, which is especially resistant to the effects of heat and chemical action.