The RPM (Revolutions per Minute) of a fan's impeller blade when operated at or near nominal voltage. This is simply a statement of raw data. Fans from multiple manufacturers cannot be compared effectively against each other using RPM data. 5 equivalent fans from 5 different manufacturers can all have the same CFM but different RPM.
Differences in aerodynamic designs have a profound impact on air performance. Stated another way: Equivalent fans from different manufacturers can have identical RPM ratings but different CFM specifications.
ORION has improved CFM specifications on several fan models simply by changing the impeller designs - in one case by as much as 12% - without changing any other specifications, including RP.