The measure of the internal friction or the resistance to flow a liquid.
Low viscosity fluids flow easily, for example, water;
High viscosity fluids pour slowly, for instance, molasses.

Measurement of a fluid's resistance to flow. The common metric unit of absolute viscosity is the poise, which is defined as the force in dynes required to move a surface one square centimeter in area past a parallel surface at a speed of one centimeter per second, with the surfaces separated by a fluid film one centimeter thick. For convenience, the CentiPoise(cP) --- one one-hundredth of a poise --- is the unit customarily used. Laboratory measurements of viscosity normally use the force of gravity to produce flow through a capillary tube(viscometer) at a controlled temperature. This measurement is called kinematic viscosity. The unit of kinematic viscosity is the stoke, expressed in square centimeters per second. The more customary unit is the centistoke(cSt) --- one one-hundredth of a stoke. Kinematic viscosity can be related to absolute viscosity by the equation:
cSt = cP/fluid density

In addition to kinematic viscosity, there are other methods for determining viscosity, including:
Saybolt Universal Viscosity
Saybolt Furol Viscosity
Engler Viscosity
Redwood Viscosity

Since viscosity varies inversely with temperature, its value is meaningless unless the temperature at which it is determined is reported.