‘Redox’ is a portmanteau of reduction and oxidation. Oxidation is the loss of electrons by a molecule, atom, or ion. Reduction, despite its name, is the gain of electrons by a molecule, atom, or ion. Therefore, redox reactions include all chemical reactions which involve a change or transfer of electrons.
Like acid-base reactions, redox reactions are also subject to the law of conservation of mass and the law of definite proportions, meaning for one atom to gain an electron another must lose one. It is because of this relationship that oxidation and reduction alone are called half-reactions since they need both halves to complete the full redox reaction.