What is the definition of a unbalanced force?

There is no such thing as "an unbalanced force". There are only forces. When two

or more separate forces act on the same object, the GROUP of forces may be balanced

or unbalanced. If the sizes and directions of all the individual forces add up to zero, then

the GROUP of forces is balanced. If one of them changes or goes away, then the GROUP

of forces is unbalanced.