Weight

The weight of an object can refer either to its mass, or to the force exerted upon it due to gravity. The first meaning of the term weight is older, and is still used in metrology and in common usage. The second meaning of the term weight is more recent, and is used mainly by scientists, and is also in common usage is well. Many scientists and people with a scientific education like to claim that only the second meaning is correct, however the historical evidence shows otherwise. (Remember that since time immemorial weight has been measured with balances, which measure mass, not force exerted due to gravity, although they require gravitiational force to operate.)

There is a related confusion that pounds are a unit of weight and kilograms are a unit of mass. Both kilograms and pounds are both measures of mass, and hence also units of weight (sense 1). The Imperial or U.S. customary unit for force, i.e. weight sense 2, is pounds-force, not pounds, although pounds-force is often abbreviated to pounds.