The party (a group or an individual) in a civil case against whom a complaint, or a petition, has been filled is called the defendant or respondent. The plaintiff or claimant, on the contrary, is the party that has brought the lawsuit to court, and the one that has to prove the case - that the defendant has indeed committed a crime or done something illegal and is therefore guilty.
The term defendant, which derives from the French "to defend," was first used in this meaning in the 14th century.