watch glass(noun)

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

crystal, watch crystal, watch glass(noun)

a protective cover that protects the face of a watch

watch glass(Noun)

An item of glass laboratory equipment consisting of a simple slightly concave circle of glass. It is a general utility item, used to hold small samples, to evaporate liquids, to cover a beaker, and so on.