In physics, the electromagnetic dual concept is based on the idea that, in the static case, electromagnetism has two separate facets: electric fields and magnetic fields.
The reason for this can ultimately be traced to special relativity, where applying the Lorentz transformation to the electric field will transform it into a magnetic field.
These are special cases of duality in mathematics.
Additionally, in a sense different from the dualities above: In the usual sense of electromagnetic duality, the (unobserved) dual of the Aharonov-Bohm effect would be the phase acquired by a hypothetical magnetic monopole traveling through electric field.
In contrast, the Aharonov–Casher effect is for a magnetic dipole (moment) in electric field, and its dual in the usual sense is for an electric dipole in magnetic field, which is distinct from the Aharonov–Bohm effect.