A document filed in court that starts a lawsuit. The party who files the complaint is called the plaintiff. The party being sued is called the defendant. In some states and in some types of legal actions, such as divorce, the complaint is called a "petition," and the person filing it is called the "petitioner." When it's filed, a complaint filing must be accompanied by a filing fee payable to the court clerk, unless the plaintiff asks a judge to waive the fee based on inability to pay. The plaintiff's complaint must be served on the defendant, who then must respond by filing a document called an answer.
The contents of a complaint will vary depending on where it's filed and what kind of case is involved. In general, though, the complaint describes: