In a Court of law, a defendant is a person( including an entity like a legal character or a company). That is accused or alleged of doing something wrong. In short, we can say that a defendant is a person to whom a plaintiff lays a claim or suit in a court of law against him. When a plaintiff lays a suit or claims against a defendant, he must prove his claim beyond the reasonable doubt against him.
A defendant is a person that the plaintiff has sued for committing wrong. If the object is criminal, then it is filed under criminal prosecution or sought some civil relief if the object is civil.