Cyberpunk is a subgenre of science fiction in a future setting that tends to focus on society as "high tech low life" featuring advanced technological and scientific achievements, such as information technology and cybernetics, juxtaposed with a degree of breakdown or radical change in the social order.
Movie: Blade Runner
Book: Neuromancer by William Gibson
Video Game: Deus Ex
High tech, low society or culture.
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It is largely dormant as a genre, not least of all because we are now in the early stages of it (when most cyberpunk stuff was written, it was all fantasy - even when Deus Ex came out) and so it isn't seen so much as fanciful as inevitable.
Don't get me wrong, I love the whole idea of giant megacities and cybernetic augmentations and everything that is involved, but it seems less relevant now, now that we already have people addicted to electronic devices, albeit ones that don't hook directly into their head via a Schreiberman Port.
Syndicate was my favourite cyberpunky thing - cyborg super-agents waging the corporate wars across city streets while vestigial governments watch helpelessly. It very much fed into the 'dying earth' from one of those Usborne Books Of The Future told me about as a kid - a heavily polluted planet kept habitable by technology as a mass exodus becomes economically viable. In a way, I suppose, it's a part of history that the lore of many spacefaring games largely skip over. They talk of a period of turmoil on Earth in the 21st and 22nd centuries but that is always overshadowed by the development of FTL drives and humanity finally freed from petty battles of the scraps remaining on Earth in favour of the riches of the cosmos.