What is the term for a single piece of glass used in a door or window?

A single piece of glass used in a door or window is referred to as a "pane." This term is commonly used in construction and building contexts to describe the individual sections of glass that are set into the frame of a window or door. Each pane can vary in size and thickness depending on the design and insulation needs of the structure.

The other terms relate to glass but are not synonymous with a single piece. "Light," for example, can refer to a section of glass in a window, but it typically describes the overall area rather than just one piece. "Glazing" refers to the process of installing glass in a frame or the material itself that fills the opening, while "sheet" generally denotes a flat piece of material, which can include glass but is too broad a term in this context. Thus, "pane" is the precise term for a single glass unit in a frame.