To the Japanese, Number 18 is known as "jinzouningen." To others (mostly in Europe it seems), she's known as C18. To me, she is considered an obvious cyborg. But to others who have only seen the American version of Dragonball Z or who are simply ignorant, she is known as an android.
Who really cares what #18 is called, though? It's just another "sub vs. dub" issue, right? It doesn't matter if you call her an android or a cyborg, right? There isn't any difference between the two anyway...right? Wrong. There is a big difference, and I am about to tell you what it is.
I'm going to tell you why this "android" thing is all wrong.
Let's begin with the Japanese term "jinzouningen," the word used to refer to all of Dr. Gero's creations in the anime and manga. Literally translated, it means "artificial human". However, the way this term is used in Japanese (chiefly sci-fi) is very board, and can describe the completely mechanical creations of #16 and #19 to the cybernetically enhanced humans of #17 and #18. There is no exact English equivalent for "jinzouningen," nor is there such a word in the English language that is so broad. For English speakers, we have the option of using "android," "cyborg," or "robot" and not much else.
Ok, now you know what "jinzouningen" means, but that still doesn't explain how #18 is a cyborg. FUNimation and Viz (god bless 'em) call her an android, don't they? How do I explain that? How can I claim something that contradicts an official source? Oh, there is plenty of proof to show for my claim, but I'll address that later. This is not a "sub versus dub" issue at all. This is an issue of common misconception and ignorance.
So why did FUNimation and Viz chose to call #17 and #18 androids if they really weren't?
Because it was convenient. As stated before, there is no one word in English that matches the broadness of "jinzouningen." The use of "android" is a compromise, a runner-up option, because while not all of Gero's creations are androids, it's just easier to refer to them all with a single term. Can you imagine what things would be like if they didn't use android? You'd probably end up reading or hearing something like
"Holy crap! It's Android 16! And Cyborg 17 and 18! And Dr. Gero's brain in the robot body! And bio-organic android Cell!! OMG!"Look at how cumbersome that is -- all that versus "it's the androids!" Viz and FUNimation saved themselves a lot of time and energy by not referring to the jinzouningen by their individual labels, especially when they are collectively referred to as the same thing in Japanese (it's all about cohesion, man). The majority of Gero's creations were androids anyway, so referring to them collectively as androids in the American versions of the anime and manga (despite not being the most accurate label) only made sense. To make up for the "android" compromise, Viz graciously noted at the beginning of any Shonen Jump volume featuring #17 and #18 that they were in fact cyborgs.
All right, that makes a lot of sense. But that still doesn't tell you how #18 is a cyborg. Aren't cyborgs and androids practically the same thing anyway?
No, they're not. The main difference is that a cyborg is a human and an android is not. What exactly is an android? It's a robot, plain and simple. What makes an android special from a basic robot is that it has a humanoid shape, rather it's shaped like a person. An android can be a quaint hostess bot with simplistic programming at the local restaurant (the 2nds, Armitage III) or it can have a complex brain and be trusted with equally complex tasks (persocons, Chobits). An android isn't necessarily constructed solely of metal parts, either. In sci-fi we see androids with artificially grown reproductive systems (Naomi Armitage, Armitage III) and living skin sitting a top metal frames (T800, The Terminator). The common link between these varied beings is that they are man-made, not found in nature. They were never human.
Cyborgs, on the other hand, are always human. A cyborg, by definition, is an enhanced human, a super man. Enhancements to the cyborg come in a variety of methods. Mechanical enhancement is the one most commonly found in sci-fi and the type most people think of first. There is also chemical enhancement, a type sci-fi super soldiers often receive (oh, and let's not forget the Juicers of the RIFTS rpg system). Some people have the bizarre idea that if a cyborg gets enough enhancements (namely mechanical ones), they will turn into an android, but this is impossible since an android is a robot. Even in instances when a human's entire body (and possibly parts of the brain) are replaced or enhanced, that being is still considered human at the base (Motoko Kusanagi, Ghost in the Shell). True, often the mystery of what humanity remains then may come in to question, and true, a "full conversion borg" may eventually cast away their humanity, but the humanity was there.
All right, so an android is a robot and cyborgs are part human. How exactly are #18 and her brother cyborgs then? What enhancements do they have?
We know that #17 and #18 are cyborgs because, frankly, we're told they are outright. Throughout the Dragonball series, the twins' humanity is mentioned several times. Not only does it come up amongst the Z warriors (chiefly by Krillin it seems), but the twins make note of it themselves.
Bulma even has the privilege of seeing their exact enhancements while reading #17's blueprints.
And just what are their enhancements? Well, let me tell you, don't believe that person telling you Gero replaced their brains with computers or gave them fully mechanical arms and legs. Whatever you hear is a lie. Nowhere is it ever mentioned exactly what it is that's replaced or enhanced in the twins. The only information we have is what Bulma reads from their blueprints, and she doesn't even say exactly what. She notes that they're enhanced "almost entirely with bio-organic components" (no metal arms there, son), and have very "few tiny cybernetic parts." (Personally, that leads me to believe that the twins had to have their bodies juiced up with chemicals and hormones [hell, throw in some nanomachines too] so they could run on par with the endless energy generators Gero put inside them.)
Other than the little blips of information scattered throughout the manga and anime, there isn't much else known about the twins' cybernetics...or the twins in general, even. We don't know their real names, ages, or much about their past. We don't even know if they know much about themselves. But by now, you should know the truth of the twins. They are cyborgs, despite the pesky manner in which most of the American audience refers to them. I'm glad you realize that now, dear reader. ♥
Of course, there are still some very silly people out there that will continue to email me, saying I'm all wrong about this. For them, and for the curious, I've collected a number of images that agree with what I say here. (Wow, look...actual evidence to back up my claims. You don't see that on many DBZ websites.)
PROOF (all taken from Shonen Jump):