The scientific principle that states for every action there is an equal and opposite reaction is known as Newton's Third Law of Motion. Formulated by Sir Isaac Newton, this law explains that when one object exerts a force on another, the second object exerts a force of equal magnitude in the opposite direction on the first object. This principle is foundational in physics and can be observed in various phenomena, from the propulsion of rockets to the movement of swimmers pushing against the water. Understanding this law helps explain the interactions between forces in nature.