When a force is exerted on a box, the equal and opposite force that the box exerts back is called the reaction force.

This is a fundamental concept in physics, known as Newton's Third Law of Motion. It states:

* For every action, there is an equal and opposite reaction.

Here's how it applies to your example:

* Action: You push on the box with a force.

* Reaction: The box pushes back on you with an equal and opposite force.

This pair of forces always acts on different objects (you and the box). It's important to understand that these forces don't cancel each other out, as they act on different objects.