Gravitational inertia, often simply referred to as inertia, is a fundamental principle of physics. It's the property of matter that causes it to resist changes in its motion. According to Newton's first law of motion, an object in motion tends to stay in motion and an object at rest tends to stay at rest unless acted upon by an external force. This tendency is due to the object's inertia, which is directly proportional to its mass. The larger the mass, the greater the inertia, and the more force it takes to change the object's motion. This principle is fundamental to our understanding of how objects move and interact in the universe.