Specific speed is the relation between the impeller’s rotation speed and the rated flow rate and head. It is the most important factor to consider in the design of the impeller.
The unit of specific speed may be different depending on the commonly used unit of measure of the region, for example,

In China, Where,
Rotation speed ( )
Rated flow rate ( )
Rated head ( )

In Japan, Where,
Rotation speed ( )
Rated flow rate ( )
Rated head ( )

The specific speed defines the type of pump, (e.g. axial, mixed, or centrifugal) where similar specific speed will have a similar impeller shape.