I think the term supercarrier was first used with the Forrestal class as that class was the first to displace 60,000 tonnes. The Midway, Yorktown and Essex class displaced less. Funny fact is that the number of aircraft carried did not increase with the introduction of super carriers. Aircraft became much bigger and heavier. To have a similair amount of airplanes they needed much bigger carriers. US fleet carriers of ww2 carried same or more planes then the Nimitz class does today :D