Definition of Fluid

• A fluid is a substance that deforms continuously in the face of tangential or shear stress, irrespective of the   magnitude of shear stress .This continuous deformation under the application of shear stress constitutes a flow.

• In this connection fluid can also be defined as the state of matter that cannot sustain any shear stress.

Fig 1.1 Shear stress on a fluid body

If a shear stress τ is applied at any location in a fluid, the element 011' which is initially at rest, will move to 022', then to 033'. Further, it moves to 044' and continues to move in a similar fashion.

In other words, the tangential stress in a fluid body depends on velocity of deformation and vanishes as this velocity approaches zero. A good example is Newton's parallel plate experiment where dependence of shear force on the velocity of deformation  was established.