In criminal cases, the person accused of the crime. In civil matters, the person or organization that is being sued.
The person against whom a lawsuit is filed is usually called the defendant. In some states, or in certain types of actions, the defendant is called the respondent. The term respondent is also used to designate the person responding to an appeal.
At common law a defendant cannot have judgment to recover a sum of money of the plaintiff, but this rule is, in some cases, altered by statute.