RN
Tag vocabularies are not translations of each other. Japanese sites categorise this material with terms drawn from ordinary Japanese, from Latin, from German, and from acronyms coined as jokes in the 1970s — and when those terms cross into English lists they get copied from list to list without anybody going back to a dictionary. The result is a small body of received wisdom, repeated everywhere, that is partly wrong.
So this is a glossary with the errors taken out. Where a common English gloss does not survive checking, that is noted rather than quietly smoothed over.
The one that is usually defined backwards
Irrumatio. It is frequently listed as meaning "anything else" — a catch-all for contact that is not oral, vaginal or anal. It does not mean that, and it is not Japanese. It comes from the Latin irrumāre and describes a specific act: the active partner thrusting into a passive mouth, which is the inverse of fellatio, where the mouth is the active party. The distinction is about who is doing the moving, and Latin had separate words for it long before anybody tagged a cartoon with either.
The category the wrong definition is reaching for does exist. In Japanese it is sumata (素股), non-penetrative contact between the thighs, a term that comes out of the sex-work trade rather than out of animation.
Terms that describe a viewer, not an act
Ryona (リョナ) is the clearest example of a Japanese porn term with no English equivalent, because it names a response rather than a scenario. It is a blend of ryōki (猟奇, a taste for the grotesque or bizarre) and onanī (オナニー, masturbation — itself borrowed from German). It denotes arousal at watching a character, usually female, be beaten or injured, and it sits at the violent end of the spectrum for exactly that reason. The occasionally repeated gloss involving a dominant partner masturbating someone is not what the word means.
Onanī on its own is just masturbation, with no implication of anything else.
Tsundere (ツンデレ) also describes a pattern rather than an act — the tsun tsun of prickliness giving way to dere dere, affection — and it is a general character-archetype term borrowed into adult work rather than one that originated there. Most tsundere material is not pornographic at all.
Body-part vocabulary, which is enormous
Japanese tagging is far more granular about anatomy than English tagging, and most of these words are unremarkable in ordinary use.
Futomomo (太もも) is simply "thigh". Oshiri (お尻) is "buttocks" and ketsu (ケツ) is its blunt slang equivalent — neither has anything to do with the moon, whatever the lists say. Hinnyū (貧乳) and bakunyū (爆乳) mark the ends of a breast-size scale that has half a dozen intermediate terms nobody bothers translating.
Mecosuji (メコスジ) is the visible cleft through tight fabric, the thing English calls camel toe. Suji (筋) means a line, streak or sinew; the "line of fiber" reading floating around English lists is a literal rendering of one dictionary sense that misses how the word is actually used.
Hiyake ato (日焼け跡) is tan lines, and it usually travels with mizugi (水着, swimwear) or sukumizu, the contracted form for school swimsuits.
Terms about form rather than content
Dōjinshi (同人誌) is the one most often mislabelled. It means self-published work — dōjin, a circle of people sharing an interest, plus shi, magazine. It says how something was produced and distributed, not what is in it. Much of it is fan work using existing characters and much of it is pornographic, but neither is part of the definition, and a great deal of dōjinshi is original and non-adult. Treating it as a synonym for parody porn is a category error that English lists make constantly.
Kemonomimi (獣耳) literally means "beast ears" and denotes an otherwise human character carrying a small animal feature — ears, sometimes a tail. The distinction from kemono, which is an animal-bodied character, is a real one that the fandom polices carefully: kemonomimi is human with an accent, kemono is animal with human traits. Collapsing the two loses the thing the word exists to specify.
Futanari (ふたなり) describes drawn characters with both sets of sexual characteristics, generally at exaggerated scale. It is a fictional trope in fictional work, and it is not a term for transgender people — some English lists present it as one, which is both inaccurate about the tag and about the people. Keeping those separate matters; there is more on the trope itself in the dedicated piece.
The relationship genres
Yuri (百合, "lily") covers romantic and sexual material between women. Yaoi covers the male equivalent, and its origin is a joke at its own expense: it is an acronym of yama nashi, ochi nashi, imi nashi — no climax, no punchline, no meaning — coined for self-published work that skipped plot in favour of the sex. The seme/uke pair labels the active and receptive roles within it.
Both categories today produce far more romance-driven than sex-driven work, and the acronym long ago stopped describing the mainstream of what it names.
Netorare (寝取られ) is the passive form of a verb meaning to have one's partner taken. The grammar is doing the work: the tag is defined from the position of the person losing rather than the person gaining, which is why the active-voice counterpart netori is a separate tag with a different audience. English cuckold vocabulary makes no such distinction.
Bukkake (ぶっかけ) is likewise ordinary Japanese — bukkakeru, to splash or dash something over — and is used unremarkably in the names of noodle dishes. Drawn work simply scales the depicted quantity past anything physical, which is generally true of the medium.
Why the errors persist
Nobody checks. A gloss enters an English list, the next list borrows it, and within a few iterations the mistranslation is better attested online than the correct one. The tags themselves are stable and mostly mundane; it is the English commentary around them that has drifted.
If you want to see how any of these are actually used rather than how they are described, the tag pages on the hentai sites and manga archives are the primary evidence, and browsing a tag for ten minutes settles more definitional arguments than any glossary will.