Viscosity is a measure of how resistant a fluid is to substances moving through them.
The simplest answer is that viscosity is a measure of how thick a fluid is. The greater the viscosity the thicker is the fluid.
Water is considered a thin or low viscosity liquid in comparison to honey, which is a thick or high viscosity fluid.
Viscosity is also a measure of how easily a fluid flows. Water, being a low viscosity liquid flows faster than honey, which is a high viscosity liquid.
To understand viscosity, you need to understand fluids, which are liquids and gases.
Properties of Newtonian (common) fluids (gases or liquids)
1. Have no fixed shape, instead they take the shape of their container.
2. Easily yield to external pressure. This means that fluids move out of the way, allowing something to move through them.
3. Free-flowing substances. This means the molecules making up a fluid can move past each other.