Non est factum: (Latin: not his deed) Defence in contract law which allows a person to avoid liability because he was mistaken about the nature of the contract. For example, a person who signs away the deed to a house, thinking that the document was only a guarantee for a debt, might be able to plead non est factum. Failure to read the terms of a contract will negate this defence.

Non-joinder: If a person who should have been a party to legal proceedings has been omitted, the court may amend the pleadings to include the non-joined party.

Nonfeasance: Not doing something that one is bound to do by law. Compare with misfeasance.

Novation: Substitution of a new contractual debt for an old debt by agreement between the debtor, the creditor and a third party who takes on responsibility for the original debt.