laboratory glassware; a shallow glass dish used as an evaporating surface or to cover a beaker

shallow glass dish used to evaporate liquids, like the glass piece covering a pocket watch

A watch glass is a circular, slightly piece of glass used in chemistry as a surface to evaporate a liquid, or as a cover for a beaker. The latter use is generally applied to prevent dust or other particles entering the beaker; the watch glass does not completely seal the beaker, and so gas exchanges still occur.

a protective cover that protects the face of a watch