Inertia is the tendency for an object to remain in motion or at rest unless it is acted upon by an outside force. In the field of physics, this concept is referred to as Newton's first law of motion.
Sir Isaac Newton's first law of motion is separated into two parts: An object will not move until a force acts upon it, and once in motion, the velocity of the object will not change until something forces it. These concepts are sometimes referred to as the Law of Inertia.
Another definition of Newton's first law of motion is that a body acted upon by no net force moves at a constant velocity with zero acceleration. The term net force refers to the total amount of force pulling down on an object. A stationary object on a floor has a gravitational force pulling downward on it, but there is also a natural force pushing up from the floor. As a result, the net force is zero, meaning the object will not move until a force acts upon it and changes its velocity.
Newton's three laws of motion were compiled in his book, "Mathematical Principles of Natural Philosophy," which was published in 1687. It was in the third edition of this work that Newton combined his laws of motion and his law of universal gravitation to explain Kepler's laws of planetary motion.