can any one help me to have simple definition for compressibility and oil compressibility and why water compressibility is neglicable and for synthetic oils what is the effect of compressibilit on them compared to wter and diesel
Compressibility can be looked at as the change in volume with change in pressure. Or, similarly, you can look at in terms of change in density with respect to pressure.
If you look at the density of air over a range of pressures, you'll see that it doesn't take much pressure to have a significant affect on the density. However water's density will only vary about 2% up to pressures of about 7000 psi.
It is a judgment call on whether or not to use the incompressible assumption or not.