Definition of Perjury
Perjury is the act of making intentionally false, misleading or incomplete statements under oath. Perjury is considered a criminal offense. Sometimes legal documents can also be signed "under penalty of perjury" which means if incomplete or false statements are submitted on the forms the person may face criminal charges.
Perjury is considered a crime because the judicial system in the United States is predicated on the assumption that testimony presented to the court is accurate and reliable. If a witness commits perjury the jurors, who relied on the testimony, may convict someone who is innocent. To commit perjury the witness must: 1) deliberately provide information which is inaccurate; 2) the evidence provided must be material the trial and affect the outcome of the trial.
The U.S. Constitution does allow, under the Fifth Amendment, the right of a witness to avoid self-incriminating testimony by "pleading the fifth." Under this condition the witness can make this choice rather than offering false testimony.