Final answer:
According to Newton's third law, the term equal and opposite refers to the reaction that occurs in response to an action. Each force is equal in magnitude and opposite in direction, and they act on different systems or bodies.
Step-by-step explanation:
Newton's third law states that for every action, there is an equal and opposite reaction. This means that whenever one body exerts a force on a second body, the second body exerts a force that is equal in magnitude and opposite in direction on the first body. These forces are equal and act on different bodies, meaning they do not cancel each other out.
The correct answer to what is equal and opposite, according to Newton's third law, is reaction. This is because the action is the force applied, and the reaction is the force that occurs in response, reflecting the mutual interactions between two bodies. Newton's laws are crucial in understanding the fundamental principles of motion, where a change in velocity indicates an acceleration, and it is the net external force that causes a change in motion, as stated by Newton's first law.