Diamagnetism is the type of magnetism occurring in substances with a small negative magnetic susceptibility and a relative permeability (μ_r ) of less than unity. It is caused by changes in the orbital motion of the electrons in the atoms of the substance and is sometimes masked by the much stronger paramagnetism and ferromagnetism. In diamagnetism, the individual induced magnetic moments oppose the external field, so there is no overall magnetic moment. There is also no temperature dependence.

Diamagnetic materials are very weakly repelled by magnetic fields. The atoms or molecules of diamagnetic materials contain no unpaired spins (lone electrons in orbitals). Examples of diamagnetic materials includes all elements with filled shells (i.e., all noble gases), molecular hydrogen (H₂), copper (Cu), bismuth (Bi), water (H₂O), sodium chloride (NaCl), and all ions of alkali metals or halogens.