[water glass] n (1612) 1: a glass vessel (as a drinking glass) for holding water
2: an instrument consisting of an open box or tube with a glass bottom used for examining objects in or under water
3: a substance that consists usu. of the silicate of sodium, is found in commerce as a glassy mass, a stony powder, or dissolved in water as a viscous syrupy liquid, and is used esp. as a cement, a protective coating, and as a fireproofing agent