As per the Classical concept,
Reduction is defined as, the elimination of oxygen or any electronegative element from a substance.
CuO(s) + H2(g) → Cu(s) + H2O(l)(removal of oxygen)
2FeCl3(aq) + H2(g) → 2FeCl2(aq) + 2HCl(aq)(removal of electronegative element, chlorine)
The addition of Hydrogen or any electropositive element to a substance.
Br2(g) + H2S(g) → 2HBr(g) + S(s)(addition of hydrogen to Br)
2HgCl2 + SnCl2(aq) → Hg2Cl2(s) + SnCl4(aq)(addition of electropositive element, mercury)
As per the Electronic Concept,
Reduction reaction involves gain of one or more electrons by an atom or an ion. It leads to the decrease in positive charge and an increase in negative charge.
Fe3+ + e- → Fe2+(decrease in positive charge from +3 to +2 for Fe)
Cl2 + 2e- → 2Cl-(increase in negative charge from 0 to -1 for Cl)