Initial Definition

Mass is a measure of how much matter there is in an object.

In our daily lives, the terms mass and weight are virtually interchangable, but in reality they are not the same thing. An object on the moon weighs less than it does on the earth, and in space is weightless. The mass of the object does not change, regardless of where it is.

Weight is a measure comprised of a combination of the mass of an object and the pull of gravity on that mass.

The units of mass in the SI system of units include grams (g) and kilograms (kg). The units of mass in the Imperial system of units include pounds (lb), ounces (oz), slugs and tons.