Main Engine Rating.. Courtesy marine insight

Effective Power: The Power available at the output side of the engine i.e. at crankshaft flange of the engine which connects it with the flywheel and rest of the intermediate shaft
Rated Power: It is the continuous effective power provided by the manufacturer of the engine for a desired or rated RPM of the crankshaft. Rated power includes the loads which acts on the engine due to auxiliary system running from the engine power
Indicated Horse Power: It is a theoretical power calculated with a formula
                                                               PxLxAxN /4500
   
Where
P- Mean indicated pressure of the cylinder

L- Stroke of the engine

A-  Cross Sectional Area of the engine cylinder

N- Speed of the engine in RPM

4500 is a constant for conversion.of kgm-min to hp metric units

              In this calculation, the frictional losses are not considered. Since it is calculated from indicated pressure of  the engine, it is called Indicated Horse Power or IHP and used for calculating mechanical efficiency of the engine

Shaft Horse Power: The power delivered by the engine to the propeller shaft is measured by an instrument known as torsion metre which is available on board.
Break Horse Power: This is the power measured at the crankshaft with the brake dynamometer and is always higher than the shaft horse power. This is because the power available at shaft accounts for frictional and mechanical losses.

Gross Power: Continuous effective power provided by the manufacturer for a given RPM using defined number of auxiliaries at normal service running condition without any overloading of the engine.

Continuous Power: It is the BHP measured at the power take off end when the engine is running at continuous safe operation range outside any time limit. This is provided by the supplier.

Overload Power: It is the power excess of effective power than the rated power for a short period of time, when the same auxiliaries are used under similar service condition for limited period.

Minimum Power: The guaranteed minimum or lower most power value by the manufacturer for an approximate crankshaft RPM is the minimum power of the engine.

Astern Output Power: The maximum power engine can generate when running in the astern direction at safe condition.
Maximum Continuous Rating or MCR: It is the maximum power output engine can produce while running continuously at safe limits and conditions.

Standard Rating: This is the power output of the engine at normal service speed which gives the highest economical efficiency, thermal and mechanical efficiency. At this speed, the wear down of the engine is at the minimum rate.