Viscosity is the property of resisting uniform non-turbulent flow within the mass of a material.
It is expressed by a coefficient representing the shear stress required to produce a flow velocity gradient of one unit in the material.
By simply observing the flowing of a fluid such as water or air we see its greater or lesser propensity to proceed in a coordinated movement rather than in separate packets: the fluid is said to have a higher or lower viscosity. This evocative term thus involves kinematic properties (related to the displacement of the fluid) and physical properties (related to the nature of the fluid) acting together to describe one of the aspects of the behaviour of a fluid in relative motion.
In physics, to take into account the effect of viscosity on the movement of a fluid in general, and that of the viscosity of a current in the atmosphere in particular, we add to the external forces acting on the fluid displacement — its weight, various inertial forces etc. — a force analogous to friction; at every point in the moving fluid, the greater the viscosity, the greater this force. The intensity of this force involves a "coefficient of viscosity" which expresses the viscosity of the fluid and which is considered as representing the fluid flow conditions. Knowing this coefficient together with the order of magnitude of the dimension and velocity of the fluid flow then enables an estimation of a dimensionless number denoted Re and called the Reynolds number — from the name of the English engineer Osborne Reynolds (1842-1912) — of which the value determines whether a fluid will keep cohesive laminar flow or will break up into turbulent flow.