for Timbo it's got to be:

A term included in the call sign of large aircraft for purposes of air traffic control.

For example, air traffic control could say "Flight 277 HEAVY, come to heading 230 flight level 320 for traffic." referring to a plane having the capability of a gross weight in excess of 255,000 pounds (115,700 kg) whether or not they are operating at this weight during a particular phase of flight.