Heavy
From the FAA 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.
Can't find a reference for the ICAO (300,000 lbs) definition though...but a KC-135 is.