In law, an affirmation is a substitution for an oath when a person's religious beliefs forbid taking an oath. A lie after an affirmation is still perjury.
An affirmation is a declaration made by a witness in court pledging his readiness to speak the truth. Affirmation does not involve the taking of an oath but the witness is nevertheless bound to speak the truth. In practice, this only requires the witness to raise his hand without using any religious book to swear. This is usually allowed if a witness's religious belief prohibits the taking of an oath.