Which term describes the tendency of a moving object to continue moving at the same velocity unless an unbalanced force acts on it? a inertia?

Your initial guess is correct. The ability of an object to continue movie unless an external force is applied is in fact, inertia. This was first described by Sir Isaac Newton as his "First Law of Motion" that states: "An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force." 

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