A quantitative measure of the resistance of an object to attempts to change its velocity. Mass is often defined as a measure of the quantity of matter in a body. _Gravitational Mass_, which appears in the Law of Gravitation, is identical to this _Inertial Mass_ . A proper explanation of this involves the _Principle of Equivalence_ and _General Relativity_, and is beyond the scope of this introductory wiki. The interested student is directed toward books on relativity, or to look up these terms on the internet.