Non Est Factum Definition:

Latin for "not his deed".

 A special defense in contract law to allow a person to avoid having to respect a contract that she or he signed because of certain reasons such as a mistake as to the kind of contract.

For example, a person who signs away the deed to a house, thinking that the document signed was only a guarantee for another person's debt, might be able to plead non est factum in a court and on that basis get the court to void the contract.

This topic of contract law is discussed at length in Duhaime's Contract Law, Chapter 5.