In a criminal case, the two sides appearing before the judge are "the prosecution", which is a government department, and "the accused". In a civil case, the two sides are "the plaintiff" and "the defendant".

First a person is accused of doing something against the law, so they are "charged" with breaking/not complying with a specific law. Then, when they go to court, they are the defendant (they must try to defend themselves against the charges). So the term defendant only applies/is only used when the person who was charged goes to court.

Two useful glossaries of legal terms: The US: http://www.uscourts.gov/Common/Glossary.aspx Britain: http://www.routledge.com/cw/slapper-9780415639989/s1/glossary/