What is an unbalanced force

There's no such thing as "an unbalanced force".

If all of the forces acting on an object all add up to zero, then we say that
the group of forces is balanced.  When that happens, the group of forces
has the same effect on the object as if there were no forces on it at all. 

An example: 
Two people with exactly equal strength are having a tug-of-war.  They pull
with equal force in opposite directions.  Each person is sweating and straining,
grunting and groaning, and exerting tremendous force.  But their forces add up
to zero, and the rope goes nowhere.  The group of forces on the rope is balanced.

On the other hand, if one of the offensive linemen is pulling on one end of
the rope, and one of the cheerleaders is pulling on the other end, then their
forces don't add up to zero, because even though they're opposite, they're
not equal.  The group of forces is unbalanced, and the rope moves.

A group of forces is either balanced or unbalanced.  A single force isn't.