The third law of motion, formulated by Isaac Newton, states that for every action, there is an equal and opposite reaction. This means that when one object exerts a force on another, the second object exerts a force of the same magnitude but in the opposite direction on the first object. This principle can be observed in everyday situations, such as when you push against a wall. While you apply force to the wall, the wall pushes back with an equal force, preventing you from moving through it. This law is fundamental in understanding how forces interact in the physical world.