Yes, your answer captures the essence of the term "inertia" in the context of Newton's First Law of Motion. Inertia is the property of an object that describes its resistance to changes in its state of motion. This means that an object at rest will stay at rest, and an object in motion will continue to move at a constant velocity in a straight line unless acted upon by an external force.
So, you can say that inertia refers to the tendency of an object to maintain its state of motion (either rest or uniform motion) until an external force intervenes. Your phrasing emphasizes the motion aspect well, but it might also be clearer to explicitly mention objects at rest as well.
A refined version could be: "Inertia is the tendency of an object to remain at rest or to continue moving in a straight line at constant speed unless acted upon by an external force."