Definition of fluid What's fluid?
The definition, I read isn't clear to me,it says,

"A fluid is a substance which will yield to any continued shearing stress however small."

But, what's shearing stress? What does yield mean here?
I need a simple definition of fluid.
 A: The definition you quote essentially means "something that you can stir".
Shear stress is like pressure, though it's taken not normal to the surface of the fluid (that'd be pressure) but coplanar to the object's cross section:

In image above: force $\perp$ to $A_0$ leads to pressure (left), whereas pressure \parallel with $A_0$ leads to shear stress (right).
EXAMPLE: put a wooden spoon in a contained with water. Can you stir it$^\dagger$ (i.e. exert shear stress)? Yes? That's a fluid.

$^\dagger$: for any infinitesimally small force you put in. You will eventually also stir solids, given enough force.
A: The description of shear force and stress given by @SuperCiocia is accurate. If the material shown is a solid, there may be strain as shown by the angle, but unless the shear force and stress is large enough, the strain will be limited in the case of a solid.
In the case of a fluid, the strain is not normally limited. Adjacent layers of the fluid parallel to the force F will continue to move. As @G.Smith pointed out, a fluid is something that flows.
Now the resistance to shear in the case of a fluid is sometimes referred to as viscosity. It is kind of related to the thickness of the fluid. Although, as indicated in the definition you gave, a substance will eventually yield (flow) to any continuing shearing stress no matter how small, it may yield very slowly. Consider thick molasses on a plate. If you tilt the plate the molasses will slide down the plate due to the shearing force of gravity. But if you tilt a very little so that the component of the gravitational force acting parallel to the plate is very little, it may take a long time for the molasses to flow.
The Heinz catsup company had a slogan regarding how thick its catsup is. The ad says, "the best things come to those who wait".
Hope this helps.
