Mass is an intrinsic physical quantity of a body which in modern physics is generally defined as the strength of an object's gravitational attraction to other bodies. The unit of mass in the International System of Units (SI) is the kilogram (kg). Mass can also be understood as a measure of the energy content of a body (see E=mc²).

Mass can be experimentally defined as a measure of the body's inertia, meaning the resistance to acceleration (change of velocity) when a net force is applied. 1

In physics, mass is not the same as weight, even though mass is often determined by measuring the object's weight using a spring scale, rather than balance scale 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 the Standard Model of physics, the mass of elementary particles is believed to be a result of their coupling with the Higgs boson in what is known as the Brout–Englert–Higgs mechanism. 2