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I have been trying to search for the Romaji version of "cube" without too much success. There seem to be several variations on the word, although that is true with almost any version of a Japanese word in any form. I have so far seen sanjou, rippou, and even kyuubu. Am I entirely off track?

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三乗【さんじょう】 sanjou and 立方【りっぽう】 rippou mean "cube" as in "raise to the third power". If you want to talk about cubing numbers, you should say something like 「4の三乗は64」, which means "the cube of 4 is 64". 立方 is used in constructions like 「立方メートル」 "cubic meter" and 「立方数」 "perfect cube [number]".

キューブ kyuubu means "cube" as in "a polyhedron with six square faces", e.g. 「アイスキューブ」 "ice cube". I don't think you would use キューブ by itself to refer to the polyhedron; it only seems to be used in loaned compound constructions like 「ルービックキューブ」. Google Image searching for just キューブ appears to mostly return pictures of the Nissan Cube, for what that's worth.

Other related words: 立方体【りっぽうたい】 also means "cube" as in "a polyhedron with six square faces". In mathematical parlance, you would use 立方体 rather than キューブ. If you want a very technical way of referring to a cube in the polyhedron sense, you can also say 正六面体【せいろくめんたい】, which appears to be a calque of "regular hexahedron". Of course, people don't really say "regular hexahedron" in English, and it seems like 正六面体 is similarly uncommon in Japanese.

So, to answer your title question - which word is "proper" depends on context.

  • Good answer, though I think it is quite unclear what the questioner wants to know. You have used so-called "waapuro romaji"; perhaps the question is really about different romanisations of キューブ, such as <i>kyūbu</i> (Hepburn) or the same with a circumflex for <i>kunreisiki</i>. And 正六面体 seems to be more common than you suggest; see the Wikipedia article which redirects from 立方体. – Brian Chandler Dec 5 '14 at 5:40
  • @BrianChandler I doubt the OP wants to know about different romanizations... I think even a novice would be able to tell that e.g. sanjou and kyuubu are different words (rather than variant romanizations of the same word). Good point on 正六面体; I'll look into that some more and edit my answer. – senshin Dec 5 '14 at 15:39
  • I work in a technical field involving geometry, and I don't think "regular hexahedron" sounds all that out-of-place, especially if being contrasted with other types of hexahedrons. I'll agree that most people wouldn't know what that is, and I wouldn't expect to hear it outside of a work context. – AHelps Dec 5 '14 at 20:09
  • In English "regular hexahedron" is very unusual (this software thinks it's a spelling mistake), but in Japanese the Platonic solids can nicely be referred to as 正n面体, for n=4, 6, 8, 12, and 20. I don't think this can be described as a 'calque' either; it's normal terminology for geometric solids. – Brian Chandler Dec 19 '14 at 9:24

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