Here's what the Liddell-Scott monster dictionary has to say:
http://www.perseus.tufts.edu/hopper/text.jsp?doc=+i%29%2Fon&fromdoc=Perseus%3Atext%3A1999.04.0057
ion is your word. I'd suspect that Greek and Latin both got the word from the same Indo-European source,instead of Latin borrowing the word from Greek.