Русско-англо-русский юридический словарь

A written contract as the name suggests, is a contract whose terms have been reduced to writing. Written contracts are also commonly signed. However, a written contract may consist of an exchange of correspondence, a letter written by the promisee and assented to by the promisor without signature, or even a memorandum or printed document not signed by either party. (definitions.uslegal.com)

That is not to say that a written contract will always be clearer than a verbal contract. A poorly drafted contract can often present as many difficulties in relation to ambiguity and certainty as a verbal contract can. (The Independent)