WordWeb Online

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"
    
2. Using speech rather than writing
   - oral
    
3. Said or done without having been planned or written in advance
   - ad-lib, spontaneous

See also: common-law, spoken, unscripted

Antonym: written

Encyclopedia: Unwritten