kilogram per second

Mass flow in kilograms per second is what an engine actually cares about. A shower drains about 0.1 kg/s, a fire pumper's master stream around 30, and a large turbofan at takeoff swallows air by the tonne per second. The unit's advantage is stated in the definition above: a kilogram of air is a kilogram of oxygen-carrying matter whether the day is hot or the airfield is high, while the same statement in cubic metres changes with the weather.

Rocketry is the trade where this unit sits in the headline equation. Thrust is mass flow times exhaust velocity, $F = \dot{m} v_e$, which the site's rocket thrust solver computes directly: there is no way to push harder except to throw more mass each second or throw it faster. The Saturn V's first stage made its roughly 34 meganewtons by burning about 13 tonnes of kerosene and oxygen every second, emptying over two thousand tonnes of propellant in about two and a half minutes. Thirteen thousand kilograms per second through five engine bells is a better statement of what this unit can mean than any definition.

For liquids the bridge to the volumetric family is density, $\dot{m} = \rho Q$, and for water the numbers collapse pleasantly: 1 kg/s and 1 L/s are the same stream to within a fraction of a percent, which is why metric process engineers slide between the two without ceremony.