What is Non Est Factum under English contract law?

"Non est factum" is a defense in English contract law which translates to "it is not my deed." This defense is raised by a party who claims that they did not understand the nature or character of a contract they signed and that they should not be bound by it. The defense of non est factum is typically used when a person has been induced to sign a document through fraud, mistake, duress, undue influence, or under a misapprehension as to its nature. To succeed in this defense, the party must prove that they did not have the knowledge or understanding of the true nature of the document they signed. In summary, the defense of non est factum is a plea in English contract law that a party should not be bound by a contract because they did not understand the nature or character of the document they signed, usually due to mistake, fraud, duress, undue influence or misapprehension.