A cyborg is a human-machine hybrid; "cyborg" is a contraction of "cybernetic organism." In the strictest sense, anyone with a medical implant (such as an artificial pacemaker or cochlear implant) or prosthesis (even including contact lenses or a walking stick), falls into this category. But the term is usually reserved for speculative creatures that might exist in the future on Earth or already elsewhere. The concept of a human-cybernetic synergy was popularized in David Rorvik's As Man Becomes Machine (1971).1 For more about the Star Trek variety of cyborg, the Borg (as illustrated), see hive intelligence.
1. Rorvik, David M. As Man Becomes Machine: The Evolution of the Cyborg. Garden City, N.Y.: Doubleday & Co. Inc (1971).