I've seen a spinning Amiga ball
and that was the demoscene
'Demoscene' is the word that encloses many meanings: an underground movement of people, a community of coders, graphicians, musicians, swappers, trolls, frequently grouped in 'demogroups' and gathered in scenic meetings called 'parties'.
But demoscene is much more than this. You can't define demoscene: you should see it yourself.