A counter-claim or cross-demand ; a claim or demand which the defendant in an action sets off against the claim of the plaintiff, as being his due, whereby he may extinguish the plaintiff’s demand, either in whole or in part according to the amount of the set-off. Set-off is a defense which goes not to the Justice of the plaintiffs demand, but sets up a demand against the plaintiff to counterbalance his in whole or in part Code Ga. 1882,