According to Newton, an object will stay at rest or stay in motion (i.e. maintain its velocity) unless acted on by a net external force, whether it results from gravity, friction, contact, or some other force.
Inertia is the name for the tendency of an object in motion to remain in motion, or an object at rest to remain at rest unless acted upon by a force. This concept was quantified in Newton's First Law of Motion. The word inertia came from the Latin word iners, which means idle or lazy and was first used by Johannes Kepler.
