In the States now, a "Heavy" aircraft is considered any aircraft capable of takeoff weights of more than 255,000 pounds.
FAA requirement to use the word "Heavy" in the U.S. as follows:
2-4-14. WORDS AND PHRASES
b. The word "heavy" shall be used as part of the identification of heavy jet aircraft as follows:
TERMINAL. In all communications with or about heavy jet aircraft.
EN ROUTE. The use of the word heavy may be omitted except as follows:
1. In communications with a terminal facility about heavy jet operations.
2. In communications with or about heavy jet aircraft with regard to an airport where the en route center is providing approach control service.
3. In communications with or about heavy jet aircraft when the separation from a following aircraft may become less than 5 miles by approved procedure.
4. When issuing traffic advisories.
EXAMPLE-
"United Fifty-Eight Heavy."
NOTE-
Most airlines will use the word "heavy" following the company prefix and flight number when establishing communications or when changing frequencies within a terminal facility's area.