Newton’s first law: If a body is in a state of rest, then it will remain in that state, and if the body is in a state of motion, then it will continue moving with the same velocity in the same direction unless an external force is applied on it.
The property of an object by virtue of which it neither changes its state nor tends to change the state is called inertia.
A force is an external cause that tends to change the state of rest or the state of motion of an object.