Mass is a fundamental physical property defining the quantity of matter contained within an object or system. It is a measure of an object's resistance to changes in motion, a characteristic known as inertia, and is measured in standard units of the International System of Units (SI). In classical mechanics, mass is directly proportional to the force required to accelerate an object and is crucial in calculating gravitational attraction between two bodies. When dealing with relativistic physics, mass is also related to the energy contained within the object, demonstrating that mass and energy are interconnected properties of matter.