The law saying that every action has an equal and opposite reaction is Newton’s third law of motion. More precisely, when one body exerts a force on another, the second body exerts a force of equal magnitude in the opposite direction on the first.
Newton presented the law in his 1687 Principia. The paired forces always act on different objects, which is the key detail many short versions leave out. For example, Earth pulls downward on a book, while the book pulls upward on Earth with an equal gravitational force. The table’s upward support force is a different force pair.
The third law helps explain walking, swimming and rocket flight. A foot pushes the ground backward, and the ground pushes the foot forward. A rocket ejects exhaust gases backward, and the gases exert a forward force on the rocket. In ordinary Newtonian mechanics, these paired forces are closely tied to conservation of momentum, although modern physics must include field momentum in some electromagnetic and relativistic situations.