What is the tendency of a moving object to remain in unchanging motion called?

The tendency of a moving object to remain in unchanging motion is known as inertia. This concept is rooted in Newton's first law of motion, which states that an object at rest will stay at rest, and an object in motion will remain in motion with the same speed and direction unless acted upon by a net external force. Inertia is essentially a property of matter related to its mass; objects with greater mass have a greater tendency to resist changes in their state of motion.

In the context of this question, options like acceleration refer to the change in velocity of an object due to an external force, while force is an interaction that can cause a change in an object's motion. Friction is a force that opposes motion between surfaces in contact. None of these terms accurately captures the idea of an object's natural resistance to changes in its motion, which is the essence of inertia.