ON WHICH PRINCIPLE?

The principle of redox is the transfer of electrons between two chemical species. It therefore depends on their oxidation states.
Oxidation state:
A chemical element has the oxidation state (or oxidation number):
n⁺ if it has lost n electrons
n^- if it has gained n electrons

Definition:
1) Oxidation is a loss of electron(s): the element’s oxidation state increases. The substance that loses electrons is called a reductant
e.g. ferrous iron Fe²⁺ is oxidized to ferric iron Fe³⁺ by losing one electron.
2) Reduction is a gain in electron(s): the element’s oxidation state decreases. The substance that gains electrons is called an oxidizer
e.g. mercuric mercury Hg²⁺ turns into mercurous mercury Hg⁺ in what corresponds to a reduction (gain of one electron)
Generally speaking the relationship between oxidizers and reductants is as follows: