1. Someone who deliberately gives false testimony in a court of law.
2. A person who intentionally lies while under an oath administered by a notary public, court clerk, or other official, and thus commits the crime of perjury.

A perjurer may commit perjury in oral testimony or by signing, or acknowledging, a written legal document; such as, an affidavit, declaration under penalty of perjury, deed, license application, or tax return, while knowing that the document contains false information.