What are glass panels in the frame used to make a window referred to as?

The term used to refer to glass panels within a window frame is "glazing." Glazing specifically denotes the act or process of fitting glass into a window frame, as well as the glass itself that is used in that context. In construction and carpentry, proper glazing is essential for ensuring good insulation and weatherproofing of windows, which enhances the overall efficiency and durability of a building.

Other options presented do not match this definition. Veneers pertain to thin slices of wood used to surface furniture or cabinetry, and insulation refers to materials used to reduce heat transfer and improve energy efficiency in a building. Shutters are typically external or internal coverings that can be opened or closed to cover windows, but they do not represent the glass itself within frames. Thus, gluing together the understanding of these terms highlights why glazing is the accurate choice in this context.