All fluids are compound of small molecules which are in constant motion. The distance between molecules is very large are compare with the molecule diameter.

The molecules are not fixed in a lattice but more about freely related to each other. Therefore, fluid density, or mass per unit volume has no precise meaning because the number of molecules occupying a human volume continuously changes. These molecules agitate and collide with one another.

The average distance that a particle move before pollution is known as the mean free path. This distance is very small. It may be noted that one m^3 of air at STP (standard temperature and pressure) contains about 2.5×10^25 molecules, which have the mean free part of about 5×10^-5 mm.

Continuous distribution of a matter which no voids are empty space is called as continuum.

 example :water flowing through quite channel.

In  most engineering applications, the size of the passage through which the fluid flows is very large in comparison with the size of the molecules and the mean free path. The average effects on of the moment of the molecules on the properties of the fluid almost constant. The engineer is generally interested in the average of macroscopic effects of my molecules. For all practical purposes, the fluid is awesome us continuum,i.e. a continuum medium. Fluid properties such as mass density ,pressure, velocity, temperature, are assume to have definite value at each point of the continuum. This property for considered to be continuous function of space and time.

However ,the concept of continuum is not valid for gases at a very low pressure.Because the distribution of matter is not continuous,i.e. continuum does not exist.

Though the concept of continuum is arbitrary, it is very useful in simplifying the mathematical analysis .Without the concept of continuum one would have to take into the account the action of each molecule or group of molecules separately in a fluid flow.

Types of fluid 

●)Ideal fluid

●)Real fluid

●) Newtonian fluid

●)Non-newtonian  fluid

●)Ideal plastic fluid

●) Compressible fluid

●) Incompressible fluid

Both incompressible and non-viscous describe an ideal fluid. Thus, a viscous liquid that has no palpable forces between its layers of liquids in motion is an ideal liquid. Since the liquid is incompressible, its density does not change even after pressure is applied to it. The irrotational nature of the ideal fluid results in its smooth flow. Due to their extremely low viscosity values, fluids like air and water might be regarded as ideal fluids