-the quantity of matter as determined from it's weight or from Newton's second law of motion


-the force that gravitation exerts upon a body, equal to the mass of the body times the local acceleration of gravity; it IS NOT mass


Let's look at an example:

This is what differentiates mass from weight: we will have the same mass anywhere in the universe regardless of what bodies we are near. However, our weight changes depending on how strong the local force of gravity is.