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Definition:

Viscosity is a measurement of how resistant a fluid is to attempts to move through it. A fluid with a low viscosity is said to be "thin," while a high viscosity fluid is said to be "thick." It is easier to move through a low viscosity fluid (like water) than a high viscosity fluid (like honey).

Most common fluids, called Newtonian fluids (yes, another thing named after that Newton), have a constant viscosity. There is a greater resistance as you increase the force, but it's a constant proportional increase. In short, a Newtonian fluid keeps acting like a fluid, no matter how much force is put into it.

In contrast, the viscosity of non-Newtonian fluids is not constant, but rather varies greatly depending on the force applied. A classic example of a non-Newtonian is Oobleck, which exhibits solid-like behavior when a large amount of force is used on it. Another type of non-Newtonian fluid are known as magnetorheological fluids, which respond to magnetic fields by becoming nearly solid but reverting to their fluid state when removed from the magnetic field.

Also Known As: fluid resistance, fluid friction, internal friction
Examples:
Water has a lower viscosity, so it is thin; the viscosity of honey is higher, so it is thick.
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