An intentional lie given while under oath or in a sworn affidavit.
The US Code (as of 2007) Title 16, Part 1, Chapter 79, at ¶1621, defines perjury as follows:
"Whoever...
• ... having taken an oath before a competent tribunal, officer, or person, in any case in which a law of the United States authorizes an oath to be administered, that he will testify, declare, depose, or certify truly, or that any written testimony, declaration, deposition, or certificate by him subscribed, is true, willfully and contrary to such oath states or subscribes any material matter which he does not believe to be true; or
• in any declaration, certificate, verification, or statement ... willfully subscribes as true any material matter which he does not believe to be true;
is guilty of perjury and shall, except as otherwise expressly provided by law, be fined under this title or imprisoned not more than five years, or both. This section is applicable whether the statement or subscription is made within or without the United States."
Canada's Criminal Code (as of 2007) defines perjury at ¶131:
"(E)very one commits perjury who, with intent to mislead, makes before a person who is authorized by law to permit it to be made before him a false statement under oath or solemn affirmation, by affidavit, solemn declaration or deposition or orally, knowing that the statement is false."
Historically, the Visigoth Code described perjury as: "Concerning Those who Destroy their Souls by Perjury" adding:
"If anyone, on account of oppression of any kind, should knowingly conceal the truth or should perjure himself; as soon as the fact shall come to the knowledge of the judge, he shall be arrested; shall receive a hundred lashes; shall be branded as an infamous witness; and shall never again be permitted to testify in court. And, as has been provided by a former law relating to perjury, a fourth part of his property shall be given, by order of the judge, to him whom he attempted to defraud."