How is maximum rate of climb defined in aircraft specifications?
The "maximum rate of climb" is the highest sustainable ROC for the following conditions...


sea level conditions (or standard atmosphere for other altitudes)   
maximum gross or takeoff weight
cruise configuration
maximum rated continuous thrust/power


There are additional minor requirements given in FAR 23 such as the rated power 8 seconds (SFAR to 23, par 6 and 23.63-23.67) after the throttles are moved to the full thrust position but the above is a good generalization.