Transhumanism is the philosophy that we can and should develop to higher levels, both physically, mentally and socialy using rational methods.
Max More defined transhumanism in Lextropicon as:
Philosophies of life (such as extropian perspectives) that seek the continuation and acceleration of the evolution of intelligent life beyond its currently human form and human limitations by means of science and technology, guided by life- promoting principles and values.
Mitch Porter:
Transhumanism is the doctrine that we can and should become more than human.
Natasha Vita More:
TH is a commitment to overcoming human limits in all their forms including extending lifespan, augmenting intelligence, perpetually increasing knowledge, achieving complete control over our personalities and identities and gaining the ability to leave the planet. Transhumanists seek to achieve these goals through reason, science and technology.Robin Hanson:
Transhumanism is the idea that new technologies are likely to change the world so much in the next century or two that our descendants will in many ways no longer be "human."
See also Joost Bonsten's version of the Transhumanist Principles