Definition of Fluid: What is Shear Stress?

[Micko]

Hello,
I have found the following definition in wikipedia:
A fluid is defined as a substance that continually deforms (flows) under an applied shear stress. All liquids and all gases are fluids.
However, it's pretty hard for me to understand this definition. Shear stress represents tangential pressure applied. Fluid will also deform under perpendicular force. Why tangential force is used in definition?

Thanks

 

[pallidin]

A fluid will deform by it's own weight, which can be tangential to the surface holding the fluid.
Not sure if that answers your question though.

 

[Dale]

Fluid will also deform under perpendicular force. Why tangential force is used in definition?

Gasses will deform under perpendicular force, ie they are compressible. Liquids do not. Fluids include both liquids and gasses, so compressibility cannot be part of the definition.

Also, even in a compressible fluid, the "perpendicular" deformation is not continuous. So a normal stress does not cause a fluid to flow, only a shear stress.