Definitions

 Oxidation exactly meant 'combination with oxygen' and reduction 'removal of oxygen'. These definitions have been illustrated. Oxidation implies combination with a more electronegative element, the removal of a less electronegative one, or same the removal of electrons. Reduction is the opposite to oxidation and in general implies addition of electrons. In any reaction where one substance is oxidized, another must be reduced: the term redox reaction is used to explain this.

As the reaction of zinc (Zn) in aqueous acid,

where zinc metal is oxidized to Zn2+, and hydrogen reduced from H+ to H2; and the reduction of zinc oxide by carbon,

where zinc is reduced from ZnO to the metal, elemental carbon is oxidized to CO, and oxygen, combined with a less electronegative element on both sides, is not oxidized or reduced.