From the Pilot Controller Glossary.....
"AIRCRAFT CLASSES- For the purposes of Wake Turbulence Separation Minima, ATC classifies aircraft as Heavy, Large, and Small as follows:
a. Heavy- Aircraft capable of takeoff weights of more than 255,000 pounds whether or not they are operating at this weight during a particular phase of flight.
b. Large- Aircraft of more than 41,000 pounds, maximum certificated takeoff weight, up to 255,000 pounds.
c. Small- Aircraft of 41,000 pounds or less maximum certificated takeoff weight."
The exception to this is the 757- in the US only, it is considered a "heavy" due to the need for additional separation from other aircraft and its unique wake vorticies.
In the US, "heavy" is used in the terminal areas only- ground, tower, departure and approach. When talking to Center, you are supposed to drop the heavy. But we all mess up, controllers too, and use it at other times. Not a huge deal.