Newton's third law of motion states that "for every action, there is an equal and opposite reaction." This law is one of the fundamental principles of classical mechanics and is articulated in Isaac Newton's work "Philosophiæ Naturalis Principia Mathematica" (Mathematical Principles of Natural Philosophy), commonly known as the Principia, published in 1687.

In simpler terms, Newton's third law can be explained as follows:

1. Action: Whenever one object exerts a force on a second object, the first object is said to be exerting an "action" on the second object.

2. Reaction: In response to the action, the second object simultaneously exerts a force of equal magnitude but in the opposite direction on the first object. This force is referred to as the "reaction."

In mathematical terms, if object A exerts a force �A on BFA on B​ on object B, then object B exerts a force �B on AFB on A​ on object A, and these forces are equal in magnitude but opposite in direction:

�A on B=−�B on A.FA on B​=−FB on A