The FAA has recently produced a legal interpertation defining known ICE for flights in the USA. As most guidance from the FAA it is not very usfull, if you get in trouble on a flight in iceing conditions you get hung out to dry. If the flight is completed ok you are ok, go figure.
The FAA also defines known ice in the Aeronautical Information Manual as ice observed accumulating on the aircraft.
The link for the interpertaion is below.
Regulations