In physics, mass is a property of a physical body. It is a measure of an object's resistance to acceleration (a change in its state of motion) when a force is applied. It also determines the strength of its mutual gravitational attraction to other bodies. In the theory of relativity a related concept is the mass–energy content of a system. The SI unit of mass is the kilogram (kg).
Mass is not the same as weight, even though we often calculate an object's mass by measuring its weight with a spring scale, rather than comparing it directly with known masses. An object on the Moon would weigh less than it does on Earth because of the lower gravity, but it would still have the same mass. This is because weight is a force, while mass is the property that (along with gravity) determines the strength of this force.
In Newtonian physics, mass can be generalized as the amount of matter in an object. However, at very high speeds, special relativity postulates that energy is an additional source of mass. Thus, any stationary body having mass has an equivalent amount of energy, and all forms of energy resist acceleration by a force and have gravitational attraction. In addition, "matter" is a loosely defined term in science, and thus cannot be precisely measured.