Inertia is a commonly considered a property of mass. It is a measure of how difficult it is to accelerate (or decelerate) the object. Inertia is proportional to mass. Newton's use of the word is on the context of a principle. i.e. the principle of inertia is an object's inability to change its velocity (a vector) unless acted upon by an external force.

The principle of inertia No force is required to maintain motion with constant velocity in a straight line, and absolute motion does not cause any observable physical effects. In layman's terms: An object that is moving will keep moving until something stops it.

Inertia

inertia