Definition: when three celestial bodies form a straight line
I think we expand this to life in general. This is a reification of the idea that two things are a coincidence, but three are a pattern. You can always connect two things with a straight line, but you can only connect three things with a straight line when they form a syzygy. Thus, we discount pairs, and put more emphasis on triples.
Why I Looked It Up
A friend used the word to name a project at work. He said it was his favorite word.