watch glass

from The American Heritage® Dictionary of the English Language, 5th Edition.

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

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a protective cover that protects the face of a watch
• noun laboratory glassware; a shallow glass dish used as an evaporating surface or to cover a beaker