Aaactually... none of you answered correctly :\ hence I'm reopening it for a bit.
Xion's name is really "no i" + x. However, while "No I" is the likely meaning behind it, showing lack of identity (a replica who is seen differently by other people? perfect), the proper, confirmed meaning is No. i - the number i, being the square root of -1. An imaginary number, one defined but that doesn't really exist - much like Xion herself.