Description: For every force, there is an equal and opposite force pushing back. 
Key Terms: Action, reaction, normal force, mass, acceleration

Newton’s third law states:

Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

We can call one force the action force and the other the reaction force. Then we can express Newton’s third law in the form:

To every action there is always an opposed equal reaction.

It doesn’t matter which force we call action and which we call reaction. The important thing is that they are co-parts of a single interaction and that neither force exists without the other.