1. Mach Number: In fluid dynamics, the Mach number (M or Ma) is a dimensionless quantity representing the ratio of flow velocity past a boundary to the local speed of sound.
M = c/a
M is the Mach number, c is the local flow velocity, a is the speed of sound in the medium
2. Critical Pressure: The Pressure for which the maximum discharge through nozzle occurs is called as critical pressure . It is denoted as Pc