Fluids may be classified as follows:

• Ideal fluid

• Real fluid

• Newtonian fluid

• Non-Newtonian fluid.

**Ideal fluid**

An ideal fluid is one, which is incompressible and has no viscosity. Such a fluid is only imaginary, as all existing fluids possess some viscosity.

**Real fluid**

Simply speaking, a fluid which possesses viscosity is known as a real fluid. All fluids, in actual practice are real fluids.

**Newtonian fluid**

During the course of our discussion on viscosity, we have seen that shear stress is proportional to the velocity gradient i.e. ra dv/dy. A real fluid in which the shear stress is proportional to the velocity gradient is known as a Newtonian fluid.

**Non-Newtonian fluid**

A real fluid in which the shear stress is not proportional to the velocity gradient is known as a non-Newtonian fluid.