a question about oxidation

No.
the definition of redox is "there's change in oxidation number of elements in a reaction".

in LOK's example, gain / loss of electrons can lead to change of oxidation number.
for example, Mg ------> Mg(2+) + 2e(-)
the oxidation number of magnesium changes from 0 to +2 .

however, in some reactions, there's no electron transfer, yet the oxidation number of elements changes. this's especially common in reactions involving covalent molecules.

for example, C + O2 ------> CO2
in formation of covalent molecules, electrons are shared instead of transferred.
oxidation number of C increases from 0 to +4, while O.N. of oxygen decreases from 0 to -2.

you can see that electrons on carbon are NOT lost; it's shared with oxygen.
Yet, O.N. of C increases and is oxidized.


there're a lot more reactions of this type. for example,
oxidation from ethanol to ethanoic acid, from sulphur to sulphur dioxide, from hydrogen to hydrogen chloride (a Covalent molecule!), etc.