The courts
perjury
Lying under oath about something material, in court, in a deposition, or before Congress. A federal felony, and notoriously hard to prove.
In full
The government must show the statement was false, that the witness knew it was false, and that it mattered to the proceeding. Honest confusion, evasion, and literally true but misleading answers generally fall outside the statute. Related offenses cover false statements to federal investigators, which require no oath at all.
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A plain-language explanation for civic education, not legal advice. When in doubt, the official source named above controls.