Watch glass Meaning, Definition & Usage

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

WordNet

Definitions
  1. . (a) A concavo-convex glass for covering the face, or dial, of a watch; also called watch crystal. (b) (Naut.) A half-hour glass used to measure the time of a watch on deck. (c) (Chem.) A round concavo-convex glass of shallow depth used for certain manipulations of chemicals in a laboratory.

Webster 1913