Viscosity is an internal property of a fluid that offers resistance to flow. For example, pushing a spoon with a small force moves it easily through a bowl of water, but the same force moves mashed potatoes very slowly. In fact, one of the major differences between styles of mashed potatoes is the viscosity of the starchy mass: some people like their potatoes running and teeming with milk and butter (they are fans of low-viscosity potatoes), while others like their potatoes drier and stickier, so they almost crack rather than flow (these people are devoted to high-viscosity potatoes.)
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In ceramics, the viscosity of the fired enamel depends on its chemical composition and the firing temperature, viscosity decreases when the temperature is higher and becomes more fluid.
Alkali glazes become more fluid with increasing temperature, so that a slight over firing can make them drain, while the viscosity of the boron ones changes more slowly, making it possible to work at more varied ranges of temperatures as they hardly react to temperature differences.
If a glaze loses water on a vertical plane, this is due to its low viscosity at the firing temperature which has been chosen. Glazes which are too fluid can be made more viscous by adding quartz, feldspar, kaolin or clay.