Mass flow rate is a measurement of the amount of mass (typically expressed in kilograms or pounds) passing through a particular cross-sectional area per unit of time (typically expressed in seconds or hours). 

It is commonly used in fluid dynamics and thermodynamics to quantify the rate at which mass is moving through a system, such as fluids through pipes or gases through engines.

Mathematically, mass flow rate    is calculated using the formula:

  = ρ •  A • V

Where:

•    is the mass flow rate (in kg/s or lb/s)
•  ρ is the density of the fluid (in kg/m³ or lb/ft³)
• A is the cross-sectional area through which the fluid is flowing (in m² or ft²)
• V is the velocity of the fluid perpendicular to the cross-sectional area (in m/s or ft/s)

This formula indicates that the mass flow rate is dependent on the density of the fluid, the size of the cross-sectional area, and the velocity of the fluid. Increasing any of these factors will increase the mass flow rate.