What is Inertia?

Inertia is that property of matter by virtue of which it resists a force tending to disturb its condition as to rest or motion. It is the statement in one word of Newton's first law of motion, that a body at rest remains at rest, and a body in motion continues to move in a straight line with uniform velocity, unless acted upon by an outside force. A child on a sled, if the rope is suddenly jerked, is left sitting on the snow; or a boy on horseback, if the horse suddenly stops, pitches over its head. These Illustrate inertia.