Plain English Meaning
A word added after the call sign on the radio to tell controllers and other pilots that this is a very large aircraft. Pilots of these aircraft must say 'Heavy' so everyone knows a big jet is in the area and extra spacing is needed behind it.
Full Definition
A weight classification suffix added to an aircraft's call sign in radio communications when that aircraft is capable of takeoff weights of 300,000 pounds or more, regardless of whether it is operating at that weight during a particular flight.
Context Anchor
You may see this term in discussions of ATC radar weather displays or hear it when a controller describes the intensity of precipitation near your route.
Derivation
Heavy comes from an Old English word meaning weighty. In use, it shifted from physical weight to the idea of a large amount or strong intensity, as in heavy .