What is the definition of balanced and unbalanced force?

Balanced forces add up to make a net force of zero. Unbalanced forces have a net force that is not equal to zero, therefore causing the object which they are acting upon to accelerate.

A balanced force is when two equal forces are applied on an object pushing both ways. Also they are equal forces acting on an object in opposite directions.
When something isn't moving the forces are balanced, eg. Gravity pulls a table down but that table is pushed up by the floor or the ground it stands on so doesn't move.

An UNbalanced force is needed for something to change movement or change direction, eg. a seesaw moving up and down because the forces aren't equal.