Weight
Weight, the force exerted on an object by the gravity of a large body such as the earth. Weight is often confused with mass, the property of matter that causes it to have inertia (resistance to any change in its motion). Mass can also be thought of as the amount of matter in an object. The weight of an object is proportional to its mass; at any given place an object with twice the mass of another will have twice the weight.
The mass of an object does not change from place to place; its weight, however, will vary depending on the mass of the large body on which it is weighed and on its distance from that body's center. For example, on the moon an object weighs only about one-sixth as much as it does on the earth, and an object weighs less on the top of a mountain than it does at the mountain's base.
Weight is measured with balances and scales. It is commonly expressed in units with the same names as those used for units of mass, such as the pound. As a unit of weight, a pound is equal to the force exerted by gravity at a standard location on an object with a mass of one pound. The unit is sometimes called a pound-force to distinguish it from the unit of mass. The kilogram is a metric unit that is also commonly used to express both mass and weight. However, in the internationally accepted form of the metric system (the SI), the kilogram is used only as a unit of mass, and weight is expressed in terms of a unit of force called a newton.