Definition of water glass:
(n) :
A drinking vessel intended or used for water.
(n) :
A water gauge for a steam boiler.
(n) :
An instrument consisting of an open box or tube with a glass bottom, used for examining objects in the water, as upon the sea bottom in shallow places.
(n) :
A water clock.
(n) :
(inorganic chemistry) Any of various water-soluble silicate compounds, especially when occurring in dissolved, jelly-like form.