A fluid is substance that can flow or continually deforms under an applied shear stress. A shear stress state is where the stress is parallel to the surface of the material. The term fluid therefore includes liquids and gas.

There are of course differences between liquids and gases. Gases are easily compressed whereas liquids are nearly incompressible. A liquid has definite size but a gas expands to fill any closed vessel containing it. However, in discussing the mechanical behavior of fluids we use only the properties of liquids and gases connected with their ability to flow. Therefore, the same basic laws control the static and dynamic behavior of both liquids and gases.

Unlike solids, fluids can change their shape readily, so their mechanical behavior cannot be described in terms of mechanics of a rigid body as is the case with solids.

There is a difference in the way a force acts on a fluid as opposed to a solid. A force can be applied to a single point of a solid and be sustained by it; but a force can only be applied to and sustained by a surface of an enclosed fluid. In a fluid at rest such a force is always directed at right angles to the surface. A fluid at rest cannot sustain a tangential force; the fluid layers would simply slide over one another when subject to such a force.

It is convenient therefore to describe the force acting on a fluid by specifying the pressure P, which is defined as the magnitude of the normal force per unit surface area.

P= Force/Area

Pressure is a scalar quantity.

Fluids display properties such as:

--not resisting deformation, or resisting it only lightly (viscosity), and

--the ability to flow (also described as the ability to take on the shape of the container).This also means that all liquids have the property of fluidity.

These properties are typically a function of their inability to support a shear stress in static equilibrium.

Solids can be subjected to shear stresses, and to normal stresses both compressive and tensile. In contrast, ideal fluids can only be subjected to normal, compressive stress which is called pressure. Real fluids display viscosity and so are capable of being subjected to low levels of shear stress.

In a solid, shear stress is a function of strain, but in a fluid, shear stress is a function of strain rate. A consequence of this behavior is Pascal's law which describes the role of pressure in characterizing a fluid's state.

Depending on the relationship between shear stress, and the rate of strain, fluids can be characterized as one of the following:

Newtonian fluids : where stress is directly proportional to rate of strain

Non-Newtonian fluids : where stress is not proportional to rate of strain

Rheology is the science of deformation and flow. One common factor between solids, liquids, and all materials whose behavior is intermediate between solids and liquid is that if we apply a stress or load on any of them they will deform or strain.