In a constant gravitational field like Earth's, this force is proportional to the object's mass, and as a result the terms are often used interchangeably and indeed went historically undistinguished. The CPGM recommends that the word 'weight' be used to refer only to force, and not to mass. The verb 'to weigh' however may be used for mass determinations.
Related to the historical identification of mass and weight, the pound has been used both as a unit of mass and as a unit of force. In the United States, United Kingdom, and elsewhere, the pound is now officially defined as a unit of mass. The corresponding force is called a pound-force, and similarly the weight of a kilogram of material on Earth is called a kilogram-force. However, the use of pounds to measure forces is still common in engineering, and it occurs in derived units like p.s.i. (pounds per square inch). In most countries, scientists have adopted SI units, which use kilogram for mass and newton for force non-interchangeably.
Stated more simply, weight (on Earth) is a force we feel from the ground which is stopping us being pulled to the centre of Earth. If we are in true free-fall, we feel no weight because there is no force to stop us accelerating under gravity.
The experience of having no weight is known as weightlessness.
See also: Hebrew weights