Weight
In modern scientific usage, weight and mass are fundamentally different quantities: mass is an intrinsic property of matter, whereas weight is a "force" that results from the action of gravity on matter: it measures how strongly the force of gravity pulls on that matter. However, in most practical everyday situations the word "weight" is used when, strictly, "mass" is meant. For example, most people would say that an object "weighs one kilogram", even though the kilogram is a unit of mass.
Mass in general relativity
where "E" is the total energy of the system, "p" is the total momentum of the system and "c" is the speed of light. Concisely, in fundamental units where formula_2, the mass of a system in special relativity is the norm of its energy–momentum four vector; otherwise, it is formula_3.