Definition: Mass
The mass of a physical object is a measure of its inertia.
INTUITION
Mass is often interpreted as the “amount of stuff” an object contains, but this is wrong and can lead to many missunderstandings. One should think about it strictly as a measure of how much an object resists being accelerated. When one thinks of mass in this way, the effects of relativity are no longer as confusing.
UNIT
The SI unit for mass is the kilogram () and it is fundamental.