Weight of an object is a force acting on the object due to gravity.
State of free fall, the weight would be zero.In this sense weight, terrestrial objects can be weightless: ignoring air resistance, the famous apple falling from the tree, on its way to meet the ground near Isaac Newton, would be weightless.
Definition: W= mg/ma, where W is the weight, m the mass of the object, and g gravitational acceleration

SI Unit : Newton

• Weight is commonly measured using one of two methods. A spring scale or hydraulic or pneumatic scale measures local weight, the local force of gravity on the object (strictly apparent?weight force).

History:

• Plato described weight as the natural tendency of objects to seek their kin.
• Aristotle, weight and levity represented the tendency to restore the natural order of the basic elements: air, earth, fire and water. He ascribed absolute weight to earth and absolute levity to fire. 
• Archimedes saw weight as quality opposed to buoyancy, with the conflict between the two determining if an object sinks or floats.
• The first operational definition of weight was given by Euclid, who defined weight as: "the heaviness or lightness of one thing, compared to another, as measured by a balance. "Operational balances (rather than definitions) had, however, been around much longer.