Heavy
Yeah think i can help with this one
Officially the Prefix heavy is for an aircraft weighing more than 136,000 Kg's
Unless your realistic then an aircraft weighing on 20,000kg's is heavy too depends if your under it.
So anything over 136,000kgs is good, mainly used for the likes of aircraft like the 767-300, 747 and larger airbuses, DC10, tristars. That type of aircraft.
Also gives an indication of the wake turbulance that following aircraft are likely to en counter so ATC can maintain the required seperation
Hope this helps