Entry from OED Online

[f. cyber- after CYBERNETICS n. + SPACE n.¹] 

    The notional environment within which electronic communication occurs, esp. when represented as the inside of a computer system; space perceived as such by an observer but generated by a computer system and having no real existence; the space of virtual reality. Cf. virtual reality s.v. *VIRTUAL a. 4 g.
 
  1982 W. GIBSON in Omni July 72/2, I knew every chip in Bobby's simulator by heart; it looked like your workaday Ono-Sendai VII, the ‘Cyberspace Seven’, but I'd rebuilt it so many times that [etc.]. 1984 — Neuromancer II. iii. 52 Molly was gone when he took the trodes off, and the loft was dark. He checked the time. He'd been in cyberspace for five hours. 1991 H. RHEINGOLD Virtual Reality (1992) i. 17 Although I stayed in cyberspace for just a few minutes, that first brief flight through a computer-created universe launched me on my own odyssey to the outposts of a new scientific frontier. 1993 Guardian 18 Oct. I. 8/7 The search for a kidnapped girl from a small town in California has leapt into cyberspace as her picture criss-crosses the world's computer networks, databases and electronic mail systems.