resistance of a fluid to shear deformation

Viscosity is a physical property of fluids. It shows resistance to flow.[1] 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, and water would be much quicker.

Illustration of viscosity; the violet fluid at the bottom has a higher viscosity than the clear one above

There are two types of viscosity: dynamic viscosity, measured in pascal seconds, and kinematic viscosity, measured in metres squared per second .[2]

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.[3]

The word viscous comes from the Latin root viscum, meaning sticky.[4]


  1. Elert, Glenn (2021). "Viscosity". The Physics Hypertextbook.
  2. "Quantities and Units of Viscosity". Uniteasy.
  3. "What is Viscosity? (with pictures)". wiseGEEK.
  4. "viscous - Origin and meaning of viscous". Online Etymology Dictionary.