Viscosity is related to the internal friction between molecules in a fluid. In liquids, it depends mainly on the strength of molecular interactions (such as hydrogen bonds), while in gases it is influenced by the frequency of collisions between moving molecules. The SI unit of viscosity is the pascal-second (Pa·s), but in practice, it is often expressed in centipoise (cP), where 1 cP = 0.001 Pa·s.

A fluid like honey has high viscosity, whereas water has very low viscosity. Temperature plays an important role: in liquids, viscosity decreases with increasing temperature (molecules move more freely), while in gases, it increases (molecules move faster and collide more often).

There are Newtonian fluids, whose viscosity remains constant regardless of the applied shear rate (e.g., water, oil), and non-Newtonian fluids, whose viscosity changes with applied stress (e.g., ketchup, blood, paints).

Viscosity is crucial in many applications: from industrial plant design to pharmaceutical formulation, from studying blood flow to ensuring proper lubrication in engines. Measuring and controlling viscosity allows for the optimization of processes and the assurance of product quality.

Viscosity is a key property for understanding the behavior of fluids in motion and their interaction with surrounding environments.