A more thorough explanation:

A plea in justification is a type of defense used by a defendant in a legal case. It is an affirmative defense, which means that the defendant is asserting facts and arguments that, if true, will defeat the plaintiff's or prosecution's claim, even if all the allegations in the complaint are true.

For example, a defendant in a criminal case may use a plea in justification to argue that they acted in self-defense or that they were forced to commit the crime under duress. In a civil case, a defendant may use a plea in justification to argue that they were not responsible for the plaintiff's damages because the plaintiff was partially at fault.

The burden of proving an affirmative defense lies with the defendant. Other types of defenses include dilatory defenses, which temporarily obstruct or delay a lawsuit but do not address the merits, and real defenses, which are good against any possible claimant.