Illustration of viscosity; the violet fluid at the bottom has a higher viscosity than the clear one above
Viscosity is a physical property of fluids. It shows resistance to flow. In a simple example, water has a low viscosity, as it is "thin". Syrup and tar, on the other hand, have a high viscosity, as they are "thick". A way to test for viscosity is the speed at which the substance runs down a slope. Syrup would reach the bottom very slowly, whereas water would be a lot quicker.
There are two types of viscosity: dynamic viscosity, measured in pascal seconds, and kinematic viscosity, measured in metres per second squared.
Viscosity is used as a way to predict when volcanoes erupt. When the lava comes out very thickly (viscous), there is more chance that it will erupt violently. This is because the lava has a hard time getting out and may burst out when it can. If the lava is thin (low viscosity), then it just flows out like water.
The word viscous comes from the Latin viscum, meaning sticky.