IUPAC has published a "Comprehensive definition of the term oxidation state (IUPAC Recommendations 2016)". It is a distillation of an IUPAC technical report "Toward a comprehensive definition of oxidation state" from 2014. The current IUPAC Gold Book definition of oxidation state is:
and the term oxidation number is nearly synonymous.
The underlying principle is that the ionic charge is "the oxidation state of an atom, after ionic approximation of its bonds", where ionic approximation means, hypothesizing that all bonds are ionic. Several criteria were considered for the ionic approximation:
# Extrapolation of the bond's polarity; 
# Assignment of electrons according to the atoms contribution to the bonding Molecular orbital (MO)/ the electrons allegiance in a LCAO–MO model.
In a bond between two different elements, the bonds electrons are assigned to its main atomic contributor/higher electronegativity; in a bond between two atoms of the same element, the electrons are divided equally. This is because most electronegativity scales depend on the atoms bonding state, which makes the assignment of the oxidation state a somewhat circular argument. For example, some scales may turn out unusual oxidation states, such as −6 for platinum in , for Pauling and Mulliken scales. The dipole moments would, sometimes, also turn out abnormal oxidation numbers, such as in CO and NO, which
are oriented with their positive end towards oxygen. Therefore, this leaves the atom's contribution to the
bonding MO, the atomic-orbital energy, and from quantum-chemical calculations of charges, as the only viable criteria with cogent values for ionic approximation. However, for a simple estimate for the ionic approximation, we can use Allen electronegativities, as only that electronegativity scale is truly independent of the oxidation state, as it relates to the average valence‐electron energy of the free atom: