I can't remember the last time I've used the word rhombus or rhombi. In English (and probably Hebrew as well) it's not a frequently used word.
I'm not sure why specialised words like this are part of the course. Especially at this level of study where so many other words would be more helpful to know.
Well, on Wordcount the word rhombus is ranked the 62059th most common word in English, comparable with little gems like ferrule, "a metal cap placed around a shaft". Another user complained recently about all those usesless sentences about turtles which is ranked the 17224th, of the same calibre as ousted, "being expelled". Well, I love both mathematics and like to know the word צָב, but was flabberghasted, when asked about a bunch of darned soccer terminology ﴾͡๏̯͡๏﴿!! So I think everybody has his time of suffering, because the lessons may reflect the personal interests of the course developpers in the more advanced lessons.
Agreed Ingeborg! I am thankful they took their personal time and volunteered to make the course, I just wish they put categories such as geometry and sports in bonus skills. I wish there were bonus skills in general, but especially for things like books, slang and idioms. (I really wish this course could get the stories like French and Spanish have, but that's a separate issue).
I would love to see sport as a bonus subject...but not geometry. I was surprised to see it so early in the course, and am really not so convinced about rhombus being useful though. We often underestimate the importance of abstract ideas and knowledge in language learning. I joined Duolingo to learn Hebrew but could not resist testing myself on the languages that I already knew. So how did real life language stand up? Yes, I found out some grammatical mistakes that I had been making for forty years...also my French spelling was better than my English spelling. O.K. I knew this already. But the biggest surprise was about concrete knowledge compared to abstract knowledge. So, we fail to realise how many abstract words we know in our own language, or how useful those words are to us. If you don't believe me, ask an eight year old child...
Well, I am no expert of Arabian etymology, but Hans Wehr lists it in his Arabisches Wörterbuch amid the root ع ي ن, i.e. together with عَيْن. The verbal root, probably denominated from "eye", means "determine, specify" too, but I am not sure why the mathematical terminology was coined this way.