A redox reaction (reduction-oxidation reaction) is a chemical reaction in which there is a change in the oxidation states of atoms, meaning electrons are transferred between reactants. Oxidation: The process of losing electrons*. The species that loses electrons is said to be oxidized. Reduction: The process of gaining electrons*. The species that gains electrons is said to be reduced. Example: The reaction between magnesium metal and oxygen gas to form magnesium oxide. $$2\text{Mg(s)} + \text{O}_2\text{(g)} \to 2\text{MgO(s)}$$ Let's break this down into half-reactions showing electron transfer: Oxidation (electron loss): Magnesium atoms lose two electrons each to become magnesium ions. $$\text{Mg(s)} \to \text{Mg}^{2+}\text{(s)} + 2\text{e}^-$$ Magnesium is oxidized. Reduction (electron gain): Oxygen molecules gain four electrons (two electrons per oxygen atom) to become oxide ions. $$\text{O}_2\text{(g)} + 4\text{e}^- \to 2\text{O}^{2-}\text{(s)}$$ Oxygen is reduced. In this reaction, magnesium loses electrons (is oxidized), and oxygen gains electrons (is reduced). Therefore, it is a redox reaction.