In common usage the term "inertia" may refer to an object's "amount of resistance to change in velocity" (which is quantified by its mass), or sometimes to its
momentum, depending on the context. The term "inertia" is more properly understood as shorthand for "the principle of inertia" as described by Newton in his
First Law of Motion; that an object not subject to any net external force moves at a constant velocity. Thus an object will continue moving at its current
velocity until some force causes its speed or direction to change.