Final answer:
Inertia is the property of matter that causes a body to maintain its state of motion; it is related to mass and quantified by it.
Step-by-step explanation:
The property of matter that causes a body to continue in its state of motion unless an external force acts on it is known as inertia. Newton's first law, often referred to as the law of inertia, states that a body at rest will remain at rest, and a body in motion will continue to move at a constant velocity, unless acted upon by a net external force. The measure of an object's inertia is directly related to its mass, which is the quantity of matter within the object. Therefore, mass is the property that best quantifies an object's inertia.