Here in the USA, it is used to be to signify any aircraft with a max takeoff weight of 300,000 lbs. or more since these type aircraft were more prone to generating more pronounced wake vortex turbulence to aircraft that followed them. After many years, the figure was later revised to 255,000 lbs. Some 757s are above 255,000 and qualify, but some are below 255,000, and while they don't technically qualify as heavies, they are treated as if they were by ATC. This entails ATC increasing the spacing between all "heavies" (or the 757s being treated as such) and other aircraft operating after them to allow more time for wake vortices to dissipate.
From the AIM...
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.
There have been several threads on this in the past...
[Edited 2004-04-21 06:48:34]
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.