VISCOSITY is a measure of the resistance of a fluid to flow. This resistance arises from the attractive forces between the molecules of the fluid. A fluid will only flow if enough energy is supplied to overcome these forces.
For a body to be able to move through a fluid, the fluid has to flow around or across it.
Therefore, the energy required to move a body through a fluid is directly related to the degree to which that fluid resists flow, i.e. its viscosity.
In scientific use, viscosity is the tendency of a liquid or gas to resist, by internal friction, the relative motion of its molecules and hence any change of shape; the magnitude of this, as measured by the force per unit, area resisting a flow in which parallel layers unit distance apart have unit speed relative to one another, also called absolute or dynamic viscosity; kinematic viscosity is the dynamic viscosity divided by the density of the fluid.