Simple Definition:

Viscosity is the measure of a substance’s resistance to flow [how easily a liquid can move]. It is a physical property that describes how thick or thin a liquid is. The higher the viscosity, the thicker the liquid is, and the more it resists flowing. Honey, molasses, and motor oil are examples of viscous liquids.

Very Simple Definition:

Viscosity is a measure of how thick or thin a liquid is. A thick liquid is harder to pour than a thin one. For example, honey is thicker than water, and it flows more slowly. Honey has a higher viscosity than water.