INERTIA

Galileo was the originator who introduced Inertia. It appeared that objects that had a behavior or a property of maintaining a state of motion. Which means, there was a resistance to changes in motion. In other words, if an object was at rest it seemed to "want" to stay at rest. Galileo called it "Inertia."

MASS IS A MEASURE OF INERTIA

Newton went one step further and related the concept of inertia to something that is possible to be measured - mass. The greater the mass of an object the greater its inertia is, and other way around too.

An an example of the relationship of mass and inertia, suppose you horizontally push two different people on the swing initially at rest, one is a very large man and the other is a small child. You'd quickly find that it was more difficult to get the adult to move. That is, there would be noticeable difference in the resistance to motion between the man and child. Also, once you got them swing and then tried to stop the motions, you'd notice that the large man is a lot harder to be stoped. Being massive, the man has greater inertia than others.