Transhumanism (also known as H+ or >H) is a movement that promotes or at least discusses the enhancement of the human race through science and technology. A transhuman society would have changed itself to the point where it would challenge the standard meaning of "being human" -- removing or changing some of the disadvantages of being human (sub-century lifespan, vulnerable to hostile bacteria) and improving what the writer thinks are the good parts of being human (thinking faster than the speed of chemicals, ceaseless endurance, always-on smartphone in your head).
It's usually associated with an anticipated event called 'the Singularity' where artificial intelligence (AI) is made that can design and build smarter AIs, which in turn design and built even smarter AIs, spiralling up and out of control. It's called 'the Singularity' because the original author said things are going to get so weird so fast there's no way to tell what any human culture will be like after this event, and it will serve as an unobservable "event horizon" just as with astronomical singularities like black holes. Transhumanists are convinced this event will usher in their ideal future, and will tell you at great length they know exactly what the future will look like post-Singularity. The irony is completely lost on them. tl;dr The Singularity is to transhumanists exactly like the Rapture is for Christians.
Transhumanism threads on /tg/ sometimes get deleted by janitors, because the threads too often turn into evangelical hugboxes that have nothing to do with tabletop games. When the Eclipse Phase RPG was published, the H+ fancreatures rallied as this was a way to cloak their preaching as talking about the game setting and fluff.
Don't confuse transhumanism for cyberpunk. Transhumanism is just going beyond what nature has given us, while Cyberpunk has lots of that, it also comprises a style of world that is to one degree or another dystopian. The two are not mutually exclusive, cyberpunk corporations are often transhuman but the technology is rarely available to all, and the industries which make some transhuman may well sentence everyone else to live in a crapsack world.
What Transhumanists want you to knowEdit
As seen by recent developments (chips in heads that allow remote control of a computer, robotic exoskeletons that allow the paraplegics to walk etc), transhumanism is rooted in realistic projections of the development of technology. However, most transhumanists are often criticized for excessive amounts of idealism and enthusiasm, even technological fetishism that doesn't care about the consequences of its actions. The line of debate is usually split between those who believe a human, scientific, involvement in the further evolution of the human race is inevitable and necessary and those who find the idea of changing humanity horrific. Counter-arguments against TH usually run along the lines of "playing god", and present various bad scenarios that could happen; for instance extermination of the human race ala Terminator or a genetic divide ala Gattaca. Others present Transhumanism as a threat to democracy as the result of said divide. Transhumanists try to argue against this by claiming their work will prevent the human stupidity that causes most of our problems, but many of them forget that our knack for making new technology is what caused most of those problems in the first place. And that's before you consider that we're not even close to figuring out how we could manage to do half of the shit they're trying to accomplish. Can't exactly give yourself superpowers when you have no clue what most of our genes do, after all.
This disregard for realism is a major weakness. It doesn't take much observation to point out that around ninety percent of its followers are either 2edgy4u teenagers trawling 4chan or academics who haven't noticed that having a degree in one subject doesn't make you a master in another unrelated subject, along with an alarmingly high number of neonazis presumably tagging along on the off-chance it gets rid of that pesky untermensch. If transhumanists could sacrifice the rest of the planet to get their immortal robot bodies would they do so? Probably, because they are narcissistic, urban, technophiles who prefer an imaginary future to current reality. This makes them excellent baddies; rich, resourceful, tough, glamorous, unappealing, replaceable, and ambitious.
The event of passing into a wholly transhumanist period is often referred to as the Singularity, where the technological advances become so fast we are unable to keep track of them and take on a "life" of their own. Also called the "Nerd Rapture", Singularity is often criticized as one of the proofs that transhumanism is just an irrational wish fulfillment fantasy, not unlike religion; whereas transhumanists harbor "contempt for flesh" and want to leave their bodies or their limitations by becoming robots or uploading their consciousness in a computer, ignoring any arguments about feasibility by insisting science will eventually find a way.
Transhumanist methods or areas of research mostly include genetics, nanotechnology, robotics and cybernetics, amongst others.
While classic transhumanists are concerned with the alteration of the human condition, transhumanists that are not total faggots just want titanium endoskeletons/machinegun hands/heat vision/immortality.
Basically, if you've ever wanted to be Darth Vader or Albert Wesker, Transhumanism is the philosophy for you.