Inertia is the fundamental physical property of an object that quantifies its resistance to changes in its state of motion. This resistance dictates that an object at rest will remain at rest, and an object in motion will continue moving at a constant velocity in a straight line unless acted upon by an external net force. Mathematically, this property is directly proportional to the object's mass, meaning that more massive objects require proportionally greater forces to alter their velocity or change their direction of travel. The concept is crucial for understanding classical mechanics, providing the foundational principle for analyzing how forces cause acceleration or deceleration within a closed system.