Define the term inertia

Explanation:

Enertia is an integral part of Newton's first law of motion.

It is the tendency of an object to stay at rest or to continue moving until and unless any external unbalanced force, (like, applied force or force of tension or frictional force ) is applied to either move it from rest or change its speed(in other words, accelerate it!!).

Example below, is of ball at rest (fig1) and if this ball is moving straight on a frictionless surface(like ice) it will keep moving!! until, we push it or pull it.