I'm walking to the store the other day, along with ELI and another friend (who will acquire a pseudonym in due course), and we're talking about the concept of living spacecraft (inspired by the work of Peter F. Hamilton, Farscape, and also Etarran's suggestion that we download and watch all of LEXX). ELI was skeptical, to which I responded that a living spacecraft would likely be some sort of cyborg - not pure flesh exposed to the void of space. ELI asked for a precise definition of "cyborg", which precipitated a complete shift in the conversation as we tried to arrive at a comprehensive understanding of organic hybrids.
An easy answer would be to say "a synergy of living matter and technology", but then your great auntie's titanium hip joint puts her in a league with Seven of Nine. So you have to qualify that only performance-enhancing technology that applies. That's not bad, because it distinguishes prosthetics from cybernetics. But what if (for whatever reason) someone swapped their arms with robotic ones of equal capability. SURELY a cyborg, if not a very practical one. Maybe they just wanted to be able to say "shit, guys, I gots me some robolimbs!" or perhaps the FDA is running tests on advanced prosthetics and they volunteered to...have...their...arms...cut...off? I dunno. Then there is the question of whether or not the technology needs to be implanted as such. I have seen it argued that the reliance of modern humans on computers to assist them in their cognitive functions (as a repository of knowledge, for example) constitutes a certain level of "cyborging".
That point didn't go over very well with the other two participants in the conversation, but I thought it was an important viewpoint to consider. If taken to extremes, no human since the invention of simple tools for hunting and gathering has NOT been to some extent a cyborg - but maybe that's the truth of the matter. We have become as we are not by the strength of our arms (body part), but by the strength of our arms (weapons)!
ELI's response is that "Cyborg" refers to a much more specific combination of human and machine, and that while he agrees that humans are oftentimes inseparable from technology, it's not the same thing. His definition of "cyborg" is "incorporating technology into the body, but with two-way interaction between the person and the mechanical bits". That's pretty good: it doesn't HAVE to be performance-enhancing, but an artificial hip doesn't really count because it's not interactive. Pretty sensible. But what happens if, for example, 'technology' and 'life' start becoming less and less distinguishable? Suppose we see the eventual merger of synthetic biology ("wet nano") and "dry nano", such that implants can be made which are for all intents and purposes alive. Do we, at such time, no longer inhabit the age of cyborgs, and live instead in a time of chimeras? If we go that route, however, it would seem unlikely that very many - if any - humans would become "cyborgs" proper; I don't really expect macro-level implants to be very good at talking to the nervous system.
I think, however, that we can still feel awesome and call ourselves "cyborgs" despite being arguably comprised entirely of living matter (just some of it introduced well after birth is all!). Consider our tool use as a species: we begin using simple tools, then use these to fabricate even better tools, and so on. We have used these tools to change our environment, and to some extent, ourselves. What we anticipate is a phase in which we no longer exist separately from our tools. I propose that we define the inhabitants of this upcoming era as "cyborgs". This neatly positions "cyborg" as more or less another word for "transhuman" (and indeed if you're into transhumanism I imagine there is already precious little distinction). It also highlights the process of becoming a cyborg as more or less an evolutionary step in our ever-growing dependence upon our technology to function. On that note, perhaps the question we should be asking isn't "what is a cyborg", but "what happens when a cyborg tries to divide by 0, or has a segfault in their silicon/metal bits?". And that's to speak nothing of the myriad questions of free will one poses when considering a read-write interface to the human brain. But that's perhaps a conundrum for another time.