non est factum Syllabification: (non est fac·tum)Pronunciation: /ˌnōn ˌest ˈfaktəm/Definition of non est factum noun Law a plea that a written agreement is invalid because the defendant was mistaken about its character when signing it. Origin:Latin, literally 'it was not done'non est factum in other Oxford dictionaries Definition of non est factum in the British & World English dictionary