Oxidation state: The applied valancy (number of electrons lost/gained) of an element in a compound is called an oxidation state.
e.g.: Oxidation state of carbon (C) in carbon monoxide (CO) is +2.
Oxidation of carbon (C) in carbon dioxide (CO2) is +4.
P.S: The total oxidation of a compound is 0
|CO ||= 0 |
|C + (-2) ||= 0 |
|C -2 ||= 0 |
|C ||= +2 |