I can't prove that the term originated from Hockey's future, but
Google Trends shows no significant usage of the term until 2018.
This is how
Hockey's Future originally defined it back in 2004:
The term "one per decade" seems contradictory as we do not see a Gretzky, Lemieux, or Orr once per decade. I believe they meant that players with a generational
ceiling appear once per decade, but that not all of them hit their ceiling. If this was indeed their intention, this would align with use of the term generational; we see a player with the potential to do this once every decade, and a player actually achieve it once every generation.
What I find helps contextualize the definition is also the definition of the tier below generational:
To me, it seems that players like Jagr, Crosby, and Ovechkin all fit firmly into the 9.5 category. You can argue Crosby had 10 potential but didn't achieve it due to injuries. You can argue Ovechkin had 10 potential but didn't achieve it due to a lack of commitment to his physical fitness. I think you can argue McDavid had 10 potential and hit it; he is the clear-cut best player of the 2000s. Based on what we've seen so far from them in the NHL, if we were to grade them based on the way that HF graded prospects, I would say Celebrini and Bedard both belong in the 9 category for potential. I would maybe give Celebrini a 9.0C and Bedard a 9.0D.
The fact that a proper, rigorous approach to this debate reduces to studying the origins of the term "generational talent" is why I find it generally pointless to engage in such debates in the first place. I mean, what the hell do I care if somebody else wants to use "generational" to describe the best player in a decade while I want to use it to describe the best player in a generation? Mine is based on the dictionary and what I believe to be the place where the term originated, theirs is based on something they read online. Who cares? We're here to discuss hockey, not English or history.
I find it much more productive to compare players to their contemporaries. Going into his draft, Bedard was said to be a player that may be on the same tier as Crosby, Ovechkin, McDavid, etc. His 113 games in the NHL are already more than enough to state he clearly does not belong in that tier.
Here's a perfect example of "generational" inflation: Making up a new tier above generational (for players who were previously considered generational) so that players in the tier below generational can now be called that.