Oh come on even such a technical word could be the same as Arabic? It's so cool...mu'3áy-yan معين
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.
A diamond is a rhombus. Don't think I will ever remember unless I write it down.