What is absolute pressure? And what is gauge pressure?

(N/A) Absolute pressure is the total pressure at a point in a fluid,which includes the atmospheric pressure acting on the surface of the fluid. It is given by the sum of gauge pressure and atmospheric pressure: $P_{abs} = P_g + P_{atm}$.
Gauge pressure is the pressure measured relative to the atmospheric pressure. It is the difference between the absolute pressure and the atmospheric pressure: $P_g = P_{abs} - P_{atm}$.
In many practical applications,gauge pressure is the value read by pressure gauges,as they are calibrated to read zero at atmospheric pressure.