1. In the physical sciences, mass is a property of a particle or object that measures its inertia, that is its resistance to change its state of motion when a force is applied to it. Strictly this is inertial mass. Mass can also be defined as the property of objects that defines how they are attracted to each other under gravity, this is known as gravitational mass. Thus far all experiments have shown that inertial mass and gravitational mass are the same (to a very large accuracy) so most people don't worry about the difference.