Hentai:
Is hentai specifically a X-rated parody of a popular anime? Or can it be any type of X-rated anime? If it’s the first is there a name for other types of X-rated anime?
Dojin (Doujin?):
Is a dojin any type of manga produced by the artist himself or does it have to be an X-rated parody?
Are you looking for actual Japanese here, or how these Japanese words are used by American anime fans?
My Japanese isn’t great, but this is one of the words you pick up pretty quickly living in Japan. “Hentai” is what you’d call a guy who tries to look down women’s blouses or up their skirts on the train. The word translates almost literally as “weirdo”, but it’s more often rendered in English as “pervert” or “perverted”. It can be used in a slightly less negative sense, like “kinky”.
As far as I can tell, American anime fans just use it to mean “pornographic”.
The way the term hentai is used within the American anime fandom has little relation to the actual Japanese meaning of the word (this is true of a number of Japanese words they’ve ‘adopted’). Among fandom, hentai means any x-rated anime or manga.
A dojin (short for dojinzasshi) is a manga or magazine self-produced by a circle of people who share an interest in something. It doesn’t have to be x-rated.
Don’t forget ecchi, which is hentai x 3.
…er, so I’ve heard it said. 
Do you know where the term “etchi” comes from? Hentai starts with an “h” in English and “etchi” is how the Japanese pronounce “h”. So it’s a short-hand for hentai that’s become it’s own word. The Japanese will also say “etchi suru” which means to “do h” or just about what you’d expect that to mean in English. (I can’t type out the f-word, I just can’t) Here’s a neat etymology.
…er, so I’ve heard it said…
As for “hentai” and “doujinshi”, look 'em up in Wikipedia if you want the current Western interpretations.
Actually, AIUI hentai is simply a word meaning, literally “great weirdness.”
Certainly the word ‘hen’ means only strange, or odd. Mind you, in Japanese culture that is still a bit stronger a connotation than here in the US, but not as bad as what one gets when the modifier ‘tai’ is added.
I really can’t help you with doujin.
I imagine, as they wake up, regular dopers like sublight may be better able to address this.
Doujin is short for doujinshi, and it’s simply any fan-produced comic. I’m pretty sure Rikdo’s Excel Saga manga was based on his doujinshi Daitenzin.
I’m afraid that the above is wrong. The ‘tai’ in hentai just means form or appearance and the ‘hen’ means unusual or changed. Hentai can be used in normally to mean ‘change form’. In a sexual context it’s actually short for hentai seiyoku, I’d translate it as ‘abnormal sexual desire’, i.e., perversion. This is why it is not generally used as a general term for sex or sexual desire the way ecchi is (while tremorviolet is probably right about the origin of ecchi, it should be noted that the two are not interchangeable, with ecchi having a much more general meaning).
Doujinshi is itself short for doujinzasshi. It’s not necessarily a comic. There are doujinshi made by clubs related to SF, etc., pretty much like American fanzines.
Yeah, I’ve heard “ecchi” come up fairly often in casual conversation and sometimes even used self-referentially. I think it’s roughly equivelant to the English “horny”…in fact, I remember one Japanese friend speculating that it may have derived from “horny” (although the “hentai” origin seems more likely).
Those of you who speak better Japanese than I (not difficult!) may know more, but the only terms I’ve heard the Japanese use to refer to porn cartoons and comics are “ero-anime” and “ero-manga”. The “ero” comes from the English “erotic”.
Does the name Kokeko have a meaning?
Do you have some kind of context for it?
I think the best general translation for ecchi (when used as an adjective) is ‘dirty’, in the sense of a ‘dirty story’ (ecchi-na hanasahi) or a ‘dirty girl’ (ecchi-na ko). It’s a good translation because it can be applied to both inanimate objects and people the way ecchi is. Of course, when used as a noun or a verb it just means sex (ecchi suru).
Those are the terms I’ve heard most often. And ‘ero-hon’ is certainly the most commonly used term for pornography, I’d say. Also worth mentioning is ‘eroi’ used as an i-adjective in pretty much the same way as ecchi-na is.
Sigh. Do people ask for me when there’s a question about advertising and ad copwriting? No. Do people call my name when they have a question about wedding ceremonies? Not a peep. Has anyone even once mentioned my name when posting a query about coming to Japan to teach? Nada.
But when you’ve grown curious about just why the cartoon schoolgirl with beachball tits and cat’s ears is getting tentacles rammed into every orifice of her body, who’s the one you turn to? Sheesh.
Granted, I guess it says something that I didn’t find this thread in a vanity search.
So, on to business. The questions seem pretty well answered already. Hentai is generally used to mean “perverted” in Japanese, and typically carries a pretty negative connotation, much more so than just “kinky”.
Etchi usually means something along the lines of “horny,” “dirty,” or sometimes just “sexual.” Used with the verb suru (to do), it simply means “have sex”.
There’s also sukebe, which is a much milder word than hentai, and usually means something like “dirty,” as in “dirty old man” but would usually be said with a nudge and a grin rather than actual distaste. When being introduced to someone at a party or in a bar, if you said “I’m sukebe,” the people around you would probably laugh and think you were funny. If you said “I’m hentai,” they may get very quiet and start backing away from you. “I’m etchi” isn’t used very often as far as I know, although if someone says to you “I want to etchi,” then you know you’re not going home alone.
The videos with the tentacles and anatomical impossibilities, by the way, are usually just called “Adult anime” (adaruto anime) in Japan.
I’m not familiar with Doujin or Kokeko. To be honest, except for Miyazaki, I don’t even watch anime (adult or otherwise).
Actually, I don’t think anyone actually wanted to ask you for your insight as to why the poor dear was being abused. Just a little etymology. Honest.