The full form of ISFC is Indicated Specific Fuel Consumption

Specific fuel consumption (SFC) is one of the most important metrics employed in aviation. It is important not only in aircraft design but also in the operation of the aircraft. First and foremost, SFC indicates how efficiently a power plant converts chemical into mechanical energy. While there is usually not a great variation in SFC between engines within a specific class of power plants, there is a huge variation between the classes. Thus, piston engines are generally more efficient than turbo machinery, which is far more efficient than, say, rockets. This is important when ensuring the selection of power plant matches the mission of the airplane, although most of the time this is not a problem. The primary importance is when estimating range and endurance of the aircraft.

As discussed in Section 7.1.5, General theory of thrust generation, the operation of any mechanical engine requires chemical energy to be converted into mechanical energy, typically by an intermediary conversion to thermal energy. In conventional piston and jet engines, the chemical energy stored in the fuel is consumed in an exothermic chemical reaction (via combustion) that adds considerable thermal energy to the air, rapidly increasing pressure or volume. The change in these states is then used to move the mechanical elements of the machine.

Consider two engines, call them A and B. If engine A requires less fuel to generate a given power (or thrust for jet engines) than engine B, then we say it is the more efficient of the two. This would place engine B at a competitive disadvantage. So, efficiency clearly plays an imperative role in the marketability of an engine (just like weight).

It is crucial for the engineer to be able to compare the efficiency of different power plants for design purposes and this is accomplished by the definition of fuel consumption as the quantity of fuel burned in a unit time (lbs/hr, kg/min, etc.). This is sometimes referred to as fuel flow (FF). We then define specific fuel consumption (SFC) as the quantity of fuel burned in unit time required to produce a given engine output. SFC is a technical figure of merit that indicates how efficiently the engine converts fuel into power.