I found this definition in the Urban Dictionary, and thought it an interesting take on Vonnegut’s term, which has remained in my head for a long time:
“Think of the small group of people you have ended up running into and spending time with over the course of your life. Some may be friends, others not. A big part of Vonnegut’s Karass concept revolves around coincidence evolving beyond apophenia, into the realm of the mystical.
The author suggests a divine plan behind the interactions of those within a karass, but adds it is unlikely that it’s purpose will ever be revealed. I suspect it is, like the universe itself, recursive in function.
In summary, to begin to understand one’s karass and those who are entwined within it, simply look for a common denominator between all those you have spent substantial time with, by choice or otherwise. The collective experiences garnered through these contacts both become and determine one’s karass.”
If nothing else, it made me look up “apophenia,” an interesting one in itself.