or

In terms of Classical idea:

Oxidation: Addition of  oxygen or any other electronegative element to a substance .Conversely , Removal of Hydrogen or any other electropositive element from a substance. 

Mg (s) + S (s) ➡MgS (s)

Similarly, Reduction  is just opposite process of oxidation.

2FeCl3(aq) + H2(g)➡2FeCl2(aq) + 2HCl (aq)

In terms of Electron transfer reaction:
Oxidation: Loss of electron 
Reduction: Gain of electron 
Oxidant: Acceptor of electron
Reductant: Donor of electron 

  • Oxidation number simply denotes the oxdiation state of an element in a compound.
  • It is mainly based on basis that electron pair in a covalent bond belongs to entirely to more electronegative element.