What is perjury, and how often is it actually prosecuted?

Perjury is making a statement under oath that you know to be false, on a matter material to the proceeding. Every element carries weight. The statement must be under oath or an equivalent affirmation, which covers testimony in court, depositions, grand juries, sworn affidavits, and forms signed under penalty of perjury such as tax returns. It must be knowingly false, so a mistaken memory, an honest misunderstanding, or a technically true but misleading answer is not perjury, a distinction the Supreme Court drew explicitly in a 1973 case involving a literally accurate but evasive answer. And it must be material, meaning capable of influencing the outcome.

Federal perjury is a felony carrying up to five years, and states have comparable statutes. Prosecutions are uncommon relative to how often witnesses shade the truth, because the knowledge element is difficult to prove: prosecutors need evidence that the person knew the statement was false at the time, usually a contradictory document, recording, or prior statement, and the cases most often brought are ones where that proof exists or where the lie was central to a larger investigation. Related offenses include false statements to federal investigators, which does not require an oath and is charged more frequently, and obstruction of justice.

A practical note that follows from the elements. Witnesses who are asked a question they do not know the answer to should say so rather than guess, because I do not recall, when true, is a complete and safe answer, while a confident guess that turns out wrong invites an accusation of lying even when it was not. And a witness who realizes they have testified inaccurately can generally correct the record, and prompt correction before the falsehood has affected the proceeding is a recognized defense under the federal statute.