The Collaborative International Dictionary of English v.0.48:

Unwritten \Un*writ"ten\, a.
   1. Not written; not reduced to writing; oral; as, unwritten
      agreements.
      [1913 Webster]

   2. Containing no writing; blank; as, unwritten paper.
      [1913 Webster]

   Unwritten doctrines (Theol.), such doctrines as have been
      handed down by word of mouth; oral or traditional
      doctrines.

   Unwritten law. [Cf. L. lex non scripta.] That part of the
      law of England and of the United States which is not
      derived from express legislative enactment, or at least
      from any enactment now extant and in force as such. This
      law is now generally contained in the reports of judicial
      decisions. See Common law, under Common.

   Unwritten laws, such laws as have been handed down by
      tradition or in song. Such were the laws of the early
      nations of Europe.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unwritten
    adj 1: based on custom rather than documentation; "an unwritten
           law"; "rites...so ancient that they well might have had
           their unwritten origins in Aurignacian times"-
           J.L.T.C.Spence [ant: written]
    2: using speech rather than writing; "an oral tradition"; "an
       oral agreement" [syn: oral, unwritten]
    3: said or done without having been planned or written in
       advance; "he made a few ad-lib remarks" [syn: ad-lib,
       spontaneous, unwritten]

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "unwritten":
   acknowledged, admitted, articulated, conventional, customary,
   enunciated, established, fixed, folk, hallowed, handed down,
   heroic, hoary, immemorial, inveterate, legendary, lingual,
   linguistic, long-established, long-standing, mythological,
   nuncupative, of long standing, of the folk, oral, parol,
   prescriptive, pronounced, received, recognized, rooted, said,
   silent, sounded, speech, spoken, time-honored, traditional,
   tried and true, true-blue, undeclared, understood, unexpressed,
   unmentioned, unproclaimed, unpronounced, unpublished, unrecorded,
   unsaid, unspoken, unsung, untalked-of, untold, unuttered, unvoiced,
   uttered, venerable, verbal, viva voce, vocal, vocalized, voiced,
   voiceful, word-of-mouth, wordless, worshipful