Viscous is a strange green liquid that vaguely resembles toothpaste. Viscous flows much slower than normal elements. It doesn't burn, boil or melt, and it floats just under the surface of Water.
When mixed with another material it is useful to make them have more friction, for example making honey, or making Rigid/Wall less slippery. Counterintuitively, it also acts as a decent lubricant because it gets stuck where it is put and makes a layer between objects.
There is a special material called Anti-Viscous. This is created by setting viscosityCoefficient in the Parameters to a negative value. This makes Viscous want to flow more instead of less, and is useful for creating some very strange materials.