resistance of a fluid to flow under applied force
measured in pascal-seconds (Pa-s) or poise
water has low viscosity (~0.001 Pa-s at 20 C), honey high (~2-10 Pa-s)
blood is intermediate and exhibits shear-thinning (non-Newtonian) behavior
decreases with rising temperature in liquids, increases in gases
dynamic viscosity vs kinematic viscosity (dynamic / density)
governs lubrication, pipe flow, sedimentation, and magma movement
Reynolds number relates viscosity to flow regime: laminar vs turbulent
critical in industrial processes: coatings, food processing, petroleum extraction