slang used in the dyno room..a dynamometer is a water brake device with load sensors attached to the brake housing. the engine is started and permitted to warm to operating temperature. water is introduced to the water break as the engine RPM is increased. This is done with various percentage of loading.. ( more water to actuate the water brakes stopping mechanism)
there is a limit to the amount of braking the engine will tolerate until the engine stops. Full load is the maximum usable horsepower or torque the engine can safely " pull" without damage.
At a given speed, "full load" is the power developed under steady state conditions at wide open throttle (for a spark ignition engine) or full fuel rack position (for a diesel).