For me genius is a born gift to excel in some domain.
Where others need 10000 hours of study and work to accomplish a certain level the genius will do this in less than 1000 hours and could do it at a younger age.
In the current society having such a born gift can even work against you since the people around you where you have to interact with every day are unable to follow your train of thought, your interests etc. which results in isolation from the group. Luckily there now special schools but still my guess is that 80+ percent of born geniusses never will pop up, be it from a lack of exposure to the field where they could excel in (say a family where no-one ever played a musical instrument so a genius musical gifted child will not be exposed to this possibility) or where they will act normal to fit in.
Yes it's a really unfortunate thing. The "gifted" children are set up for a wonderful career in whatever domain they pursue. But IQ is only half predictor of success, the other is hard work. And if they are never challenged at a young age, then they are likely to struggle and drop out at end of secondary school or college.
And then there is the issue that life can generally get much harder with high IQ. People can't follow your train of thought, while those people may pick up far above average on details and have trouble with regulating that stimuli. Not to mention that a plenty chunk of the very gifted ones also struggle with neurodivergence like Autism, ADD, etc. which also often means trouble regulating emotions.
To be honest, and this is almost an anti-scientific take on it, but I can't be fussed what a proper definition of a genius. You'll now when you spot one. For me personally though, as running around in academia with plenty of very clever people, I think staying humble is one of the pristine properties after you have jumped over all the bars of master/Phd degrees, tenure tracks, etc. Some people may brag about IQ, mental arithmetic capability, recalling events, working memory, etc.etc. but for me a lot of it becomes void if you can't be a humble human that can also recognize his/her bias, domain of expertise and limits thereof, projections or expectations on/of others, etc.
It's unfortunate that a lot of the popular genius' would be disqualified by these criteria.