Nudum pactum – The term comes from Latin and means: A rule for those agreements which are called “naked”- yet without consideration, such as a unilateral undertaking, which may bind a person morally, but not under contract law, in those jurisdictions which still require consideration. These are for example, sealed, documents, bills of exchange, etc.

See also http://legal-dictionary.thefreedictionary.com/Nudum+pactum .