What is redox?

It is known that oxidation and reduction occur at the same time. Redox is defined as a type of reaction which consists of oxidation and reduction.

For example, burning magnesium reacts with sulphur dioxide gas to give magnesium oxide and sulphur.
2Mg(s) + SO2(g) → 2MgO(s) + S(g)
The conversion of Mg to MgO is oxidation, and the conversion of SO2 to S is reduction.

There are a few different theories to define oxidation and reduction. Some of them are :
(1) Oxidation is the gain of oxygen, and reduction is the loss of oxygen.
(2) Oxidation is the loss of electrons, and reduction is the gain of electrons.
(3) Oxidation number increases in oxidation, and oxidation decreases in reduction.