Oxidation

In simple terms, oxidation is the gain of oxygen.

We can say that magnesium is oxidised as it becomes magnesium oxide when it is burned in the air.

Oxidation can also be considered as part of a pair of changes that go at the same time: reduction and oxidation (redox). Reduction can be considered as the removal of oxgen from a substance. Metal oxides can be reduced to give the metal.

When one substance is oxidised, another is reduced.