Newton's laws of motion
The three laws — inertia, F = ma, and equal-and-opposite reaction — that launched classical mechanics in 1687.
Definition
Newton's laws of motion are three statements, published together in the opening pages of the Principia in 1687, that became the foundation of classical mechanics.
The first law says that a body continues in its state of rest or of uniform motion in a straight line unless acted upon by a force. Motion is not what needs explaining — only changes in motion.
The second law, F = ma, says that the net force on a body equals its mass times the acceleration it produces. Every specific dynamical problem — a swinging pendulum, a falling apple, an orbiting planet — reduces to writing down the force and applying the second law.
The third law says that every force comes paired with an equal and opposite force on a different body. Rockets, walking, swimming: all are third-law phenomena. Momentum conservation is a direct consequence.
The three laws together hold only in inertial reference frames. Einstein's theories of relativity modified but did not overthrow them.
History
Newton published the three laws at the opening of Book I of the Principia in 1687, synthesising two decades of private calculation done at Woolsthorpe during the plague years 1665–1666. From them plus universal gravitation he derived Kepler's laws, explained the tides, and predicted the return of comets.