Transhumanism is an abbreviation of the term 'transitional human', and was first coined by the futurist FM-2030 to describe the early teetering steps which we as a species are making to becoming posthuman.
Transhumanism also describes an emergent school of speculative philosophy which is predicated on the idea that the human species does not represent the endpoint of evolution but rather its beginning. It is concerned with investigation into the implications of social, technological and scientific means of overcoming physical human limitations, and is also a movement which argues the case for fundamentally altering the human condition through these means. Typically, transhumanists believe that the rapid advances in technology will lead in the foreseeable future to the creation of Artificial Intelligence beyond anything conceived of in the Turing Test, and that this will lead inexorably to radical progress in such fields as nanotechnology and sub-molecular engineering.
There are at least two transhumanist organisations in existence: The Extropy Institute (http://www.extropy.org) and the World Transhumanist Association (http://www.transhumanism.com). Another good source of information is Anders Sandberg's Transhuman Resources site (http://www.aleph.se/Trans/).