Now that one is in that as an obscure sci fi reference and should really be taken out.
It’s from the book Voyage of the Space Beagle by A. E. van Vogt which I believe to be a direct inspiration for the film Alien (although everyone involved with the movie denies this and also the book is honestly pretty bad).
Nexialism is described as being sort of the “meta science”, the idea that all other fields of science are merely a small part of a larger whole, and nexialism is the generalist study of that larger thing. Honestly not sure how you’d use it in play, since it’s either worthless or way too broad. When I was making the original Skills list I was having some trouble filling out the full 36 so some of the entries are weaker, or merely references to things, and this is one of them.