Concentricity refers to a center of circular surfaces if the diameter of a cylinder is smaller than its hight.
In this case, for example, the center of a hole must be inside a circle of the specified diameter.
This applies around a reference point in the cross section.

For coaxiality, the axis of a selected diameter must be within a tolerance zone, which is represented by a cylinder of given diameter.
The axis or directional normal must lie on a common reference axis A-B.

The rule that cylinders with diameters greater than their height are tolerated via concentricity serves to distinguish the two terms.
Otherwise, coaxiality is tolerated.

Concentricity in Injection Molding

Quality Assurance: The axis of the diameter must be within a cylinder of 0.02mm (reference axis A-B)

Related topics:

This glossary of plastic industry is provided by PLEXPERT Canada Inc.