watch glass

Short definition from Wiktionary

Noun

1. 1
   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.
2. 2
   The glass covering of the face of a watch.
3. 3
   A sandglass, hourglass.