What is the weight of an object?

Every object has a quantity known as the mass. The mass is simply the quantity of matter that it contains.

When according to the law of gravitation, a mass that is situated inside the gravitational field of the earth would experience an attractive force, a pull, as shown in fig 1 below.

Fig 1

Now this pull on the object is called the weight.

Hence the weight can be defined as the gravitational pull that the earth exerts on an object. 

Since it is gravitational pull it is a  thus a force and its unit is the Newton (N).

The weight is calculated according to the equation below:

weight = mass * acceleration due to gravity

w  = mg 

Example

A stone has a mass of 5.0 kg. Calculate it weight if the acceleration due to gravity is 9.81 m s^-2 .

Weight w = mg

               = 5.0 * 9.81
                = 49.05 N
                = 49 N