Viscosity
Definition
A fluid's resistance to flowing — its 'thickness.' Honey has high viscosity and pours slowly; water has low viscosity and pours easily.
Symbol: · Unit: pascal-second (Pa·s)
Technical definition
A measure of a fluid's internal friction: the resistance of adjacent fluid layers to sliding past one another. Dynamic viscosity μ relates shear stress to the velocity gradient (τ = μ·dv/dy); the SI unit is the pascal-second (Pa·s).
Example
Motor oil is rated by viscosity (like 10W-30) because engines need oil thick enough to cushion parts yet thin enough to flow when cold. Honey's viscosity is roughly 10,000 times that of water at room temperature.
Viscosity is not the same as density. Mercury is far denser than water but flows almost as easily — its viscosity is only about 1.5 times water's. 'Thick' in everyday speech mixes the two; in physics, resistance to flow is viscosity, heaviness per volume is density.