Viscosity: viscosity is the measure of internal resistance to motion of a fluid due to cohesion between its own molecules.
Absolute viscosity: absolute viscosity, eta is defined as the force per unit required maintained the unit velocity difference between two parallel layers of fluid unit distance apart.
Relative viscosity: relative viscosity is the ratio of the solution. To the viscosity of the solvent used.
Viscosity index: viscosity index is defined as the rate at which viscosity of the lubricants oil changes with respect to temperature.