I checked out the experts at Wikipedia to see what how it was defined:
“Before Jaws set box office records in the summer of 1975, successful films, such as Quo Vadis, The Ten Commandments, Gone With the Wind, and Ben-Hur, were called blockbusters based purely on the amount of money earned at the box office. Jaws is regarded as the first film of New Hollywood’s “blockbuster era” with its current meaning, implying a film genre.[4] It also consolidated the “summer blockbuster” trend, through which major film studios and distributors planned their entire annual marketing strategy around a big release by July 4.[5]”
So evidently the two definitions are separated by “Old Hollywood” and “New Hollywood” demarcation. New Hollywood differs in that it relies on carnival type hype.
Not necessarily carnival hype, but speed. Averaging 3.5 major releases a week they just don’t have time for movies to sit around and build an audience, they need movies to hit hard, make a bunch of money, then get the hell out of the way because even the massive multiplexes just don’t have the room for lingerers. New Hollywood is a ravenous beast that needs $100 million openings constantly.