Newton's first law of motion states, "Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it." An unbalanced (or unbalancing) force is another name for an external force.
Some examples of external forces include gravity, air resistance and radiation. Any physical force, however small or large, perturbs the uniform motion of an object. Considering that the universe is filled with forces, the notion of a state of uniform motion is more of an ideal, because every object and every particle is being acted on by any number of forces from one moment to the next.