Mass is a property of a physical object that quantifies the amount of matter and energy it contains. Unlike weight, the mass of something stays the same regardless of location. In relativity, the quantity invariant mass, which is closest in content to the classical idea of mass, also does not vary between single observers. Mass is a central concept of classical mechanics and related subjects, and also several forms of mass appear in relativistic kinematics (see mass in special relativity).

There are three different quantities called mass:

• Inertial mass is a measure of an object's resistance to changing its state of motion when a force is applied. An object with small inertial mass changes its motion more readily, and an object with large inertial mass does so less readily.
• Passive gravitational mass is a measure of the strength of an object's interaction with the gravitational field. Within the same gravitational field, an object with a smaller passive gravitational mass experiences a smaller force than an object with a larger passive gravitational mass. (This force is called the weight of the object. In informal usage, the word "weight" is often used synonymously with "mass", because the strength of the gravitational field is roughly constant everywhere on the surface of the Earth. In physics, the two terms are distinct: an object will have a larger weight if it is placed in a stronger gravitational field, but its passive gravitational mass remains unchanged.)
• Active gravitational mass is a measure of the strength of the gravitational field due to a particular object. For example, the gravitational field that one experiences on the Moon is weaker than that of the Earth because the Moon has less active gravitational mass.