- the mass of a body as determined by the second law of motion from the acceleration of the body when it is subjected to a force that is not due to gravity.
- the measure of the property of inertia.
- the mass of a body as determined by its momentum, as opposed to gravitational mass. The acceleration of a falling body is inversely proportional to its inertial mass but directly proportional to its gravitational mass: as all falling bodies have the same constant acceleration the two types of mass must be equal
