Mass
The basic SI unit of mass is the kilogram (kg). In physics, mass is not the same as weight, even though mass is often determined by measuring the object's weight using a spring scale, rather than balance scale comparing it directly with known masses. An object on the Moon would weigh less than it does on Earth because of the lower gravity, but it would still have the same mass. This is because weight is a force, while mass is the property that (along with gravity) determines the strength of this force.
Inertia
This was clear at the beginning of the 20th century, before the advent of the theory of relativity. Mass, "m", denoted something like an amount of substance or quantity of matter. At the same time, mass was the quantitative measure of inertia of a body.