watch glass

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A shallow glass dish used as a beaker cover or evaporating surface.
2. n. A concavo-convex glass or plastic disk used to cover the face of a watch.

Wiktionary

1. n. chemistry 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.

GNU Webster's 1913

1. n. (Naut.) A half-hour glass used to measure the time of a watch on deck.

WordNet 3.0

1. n. a protective cover that protects the face of a watch
2. n. laboratory glassware; a shallow glass dish used as an evaporating surface or to cover a beaker