Newton's 3rd law of motion is the " action - reaction " law .
This 3rd law states that ;
If a body A applies a force on body B then body B also applies force on body A which is equal in magnitude and opposite in direction .
In simple terms it is stated as ;
For every action there is an equivalent opposite reaction .
This is represented as ;
Example :-
When a birds flys in the air it pushes the air with it's wings downwards ( action ) . Similarly, the air pushes the bird upwards ( reaction ) .