Question

What is inertia? what gives the measure of inertia?

Answer 1

The inability of a  body to change its state of rest or state of uniform motion by itself is known as inertia.
Inertia is a measure of mass.

Answer 2

Mass is that quantity that is solely dependent upon the inertia of an object. The more inertia that an object has, the more mass that it has. A more massive object has a greater tendency to resist changes in its state of motion.