According to Schadow and McDonald,[1] metric units, in general, are those units "defined 'in the spirit' of the metric system, that emerged in late 18th century France and was rapidly adopted by scientists and engineers.
In its most restrictive interpretation, this is what may be meant when the term metric unit is used.
There are several metric systems, most of which have become disused or are still used in only niche disciplines.
Its subsystems (CGS-ESU, CGS-EMU and CGS-Gaussian) have different defining equations for their systems of quantities for defining electromagnetic quantities and hence the associated units, with CGS-Gaussian units being selected from each of the other two subsystems.
The CGS-to-SI correspondence of electromagnetic units as given was exact prior to the 2019 revision of the SI, until which the magnetic constant μ0 was defined as 4π×10−7 N⋅A−2.