Two pieces of glass that have been sealed together with a vacuum created in the center for insulation purposes. This glass rejects heat and doesn't affect the viewing quality of the window.
unit Two or more pieces of glass spaced apart and hermetically sealed to form a single-glazed unit with an air space between the glass. (Commonly called IG units.)
Two pieces of glass spaced apart by a metal strip and sealed with rubber to create a more weatherproofed joint.