Fraud upon the court is a criminal act where by the court is impaired in the impartial performance of its task. In the U.S. fraud upon the court is usually done by an officer of the court.
The following is an example of a case law defining fraud upon the court:
Fraud upon the court embraces only that species of fraud which does or attempts to, defiles the court itself, or is a fraud perpetrated by officers of the court so that the judicial machinery cannot perform in the usual manner its impartial task of adjudging cases that are presented for adjudication.[Appling v. State Farm Mut. Auto. Ins. Co., 340 F.3d 769, 781 (9th Cir. 2003)]