A rule, usually concerning social behavior, which is known by all but spoken by none. This rule is neither official nor written down. It just is.
Examples of what an Unwritten Rule is:
You do not sit next to strangers on busses/trains even if it is full - you stand.
You do not stare at people in public.
You do not tell a girl she is fat, even if she is.
You never pass a cop even if he is doing 10 under.
You do not swear in the presence of a lady.
You do not initiate unwelcomed small talk with who you are sitting next to on a plane.
You take your hat off during the national anthem.