"A consensual hallucination experienced daily by billions of legitimate operators, in every nation, by children being taught mathematical concepts. … A graphic representation of data abstracted from banks of every computer in the human system. Unthinkable complexity. Lines of light ranged in the nonspace(sic) of the mind, clusters and constellations of data. Like city lights, receding."
The word cyberspace was coined by William Gibson in the novelette Burning Chrome, but was more notably popularized by the above quote from page 69 of his latter publication, Neuromancer in 1984.