Oxidation-reduction reaction: also called a redox reaction.
They are reactions in which both oxidation(losing of electrons) and reduction(gaining of electrons) reactions take place.
For example:
2Na + Cl2 2NaCl
In this reaction, the oxidation state of sodium has changed from 0 to +1 i.e undergone oxidation, and the oxidation state of chlorine has changed from 0 to -1 i.e. undergone reduction.
Mg + Zn(NO3)2 Zn + Mg(NO3)2
In this reaction, the oxidation state of magnesium has changed from 0 to +2 i.e undergone oxidation, and the oxidation state of zinc has changed from +2 to 0 i.e. undergone reduction.
Final Answer:
In the above examples both oxidation and reduction are taking place i.e. they are redox reactions.