Energy conversion, of course, takes place when one form of energy is converted to another - heat into work, mass into heat, electricity into work, and so on. The efficiency is the ratio of the output energy to the input energy.
Thermal Efficiency, as heat is converted into work: Work(out)/Heat(in)
Combustion Efficiency, as mass is combusted, releasing heat: Heat(out)/HV(fuel)
HV is the heating value of the fuel, which is defined as the amount of heat released when a unit mass of fuel at room temperature is completely burned and the combustion products are cooled to room temperature. If the water in the combustion products is vapor, HV is low; if it is liquid, HV is high.
Generator Efficiency, as mechanical energy is converted to electrical: Power(out)/Power(in)
Motor Efficiency, as electrical energy is converted to mechanical: Energy(out)/Energy(in)
Lighting Efficacy, as electricity is converted to luminosity in a light bulb: Lumens(out)/Watts(in)
Cooking Efficiency, as energy consumed by an appliance is transferred to food: Energy(food, post-cooking)/Energy(appliance)
Noticing a trend here? E(out)/E(in).
If a device incorporates more than one of the above processes - say, a combustion process provides heat to a heat engine, which powers a generator - the total efficiency of the process can be found by multiplying the efficiencies of the sub-processes.