I appreciate the que. I have studied mechanics in uni, let me say that - pressure concept generally is the collision of material molecules , - stress is a concept in engineering,It is a vector which dimension is [Mpa:Mega pascal or pascal or lb/in^2], it could be represent by two components, the component which is normal two the surface is called normal stress or pressure & the other which is shear to surface is called shear stress, like the decomposition of forces

"Shear Stress" is a term from physics. It is defined as "the component of stress that causes parallel layers of a material to move relative to each other in their own planes." (Wiktionary) Perhaps a good example would be the stress between overlapping tectonic plates pressing in opposite directions, or between the two blades of a pair of scissors. (I am not a physicist.)

OK, so stress and pressure usually apply directly to the surface of something. Shear stress runs parallel to the surface of something. http://en.wikipedia.org/wiki/Shear_stress I see Wikipedia also has the topic in Farsi: تنش برشی The word "shear" itself refers to cutting - in this case, it's a "cutting across".