What is the name of the law that states that for every action there is an equal and opposite reaction?

The law stating that every action has an equal and opposite reaction is Newton’s third law of motion.

In its fuller form, the law says that when one body exerts a force on a second body, the second exerts a force of equal magnitude in the opposite direction on the first. The paired forces act on different objects, which is the key detail often omitted from the familiar slogan.

Isaac Newton published the law in the Principia Mathematica in 1687. It helps explain everyday events such as walking: your foot pushes the ground backward, and the ground pushes your foot forward. A rocket similarly pushes exhaust gases backward, while the gases exert a forward force on the rocket.

A classic mistake is calling a book’s downward weight and the table’s upward support force an action-reaction pair. Those forces act on the same book and balance according to Newton’s second law. The true reaction to Earth’s pull on the book is the book’s gravitational pull on Earth. In an isolated system, third-law pairs also underpin conservation of momentum.