Sir Isaac newton who originated the concept of viscosity and is often called Non- Newtonian fluid mechanics.

Relative Viscosity often refers to that of a Polymer Solution and is defined as the ratio of the viscosity of the solution to the pure solvent. It is calculated experimentally by measuring the time that it takes for the pure solvent to pass through a certain tube, in certain conditions , and comparing it with the time it takes for the solution to pass through the same tube, in the same condition. The term Apparent Viscosity is used when you calculate the viscosity of a non-Newtonian fluid by applying equations that are derived for the viscosity of a Newtonian fluid. So it is not the actual viscosity. I hope this answers your question.

Fluid mechanics can be challenging due to its complex mathematical modeling and the need to understand fluid behavior under various conditions. The concepts of viscosity, turbulence, and fluid flow can be difficult to grasp initially. However, with practice and application, mastering fluid mechanics is achievable.

Viscosity is a fluids resistance to flow. A high viscosity fluid would be thick, a low viscosity fluid would be thin.