Ok,
obviously you know better.
I translated it as 'ξεχωριστός' (special) it in connection with all these series of books, movies etc., entitled X... something, like X-man for examble: x-man refers to special people with abilities beyond human sense, meaning, extraordinary and notable.
The same applies to all X-Factor series written by Peter David.
The show 'X-Factor' it is a reality talent show -I must confess that I realy hate them- which took this particular title according to people who is directed to: 'xtraordinary, xceptional and xciting' as I said to my previous message.
[For biology, well... I just mentioned an 'expression' I have heard, I don't advocate for its validity. I was never good at biology anyway : ) ]
NT