The volume or amount of a liquid moving through a channel or pipe. It is measured in million gallons per day (MGD), gallons per day, and cubic feet per second. In most hydraulic calculations, the flow is expressed in cubic feet per second. To obtain cubic feet per second when flow is given in million gallons per day multiply by 1.55 ft3 /sec/MGD:

Source: Handbook of water and wastewater treatment plant operations

Wikipedia:  https://en.wikipedia.org/wiki/Fluid_dynamics