It stands for synchronism, which has various other forms:
What drew him [Steve Strogatz] was a pattern in Nature that showed, surprisingly, that an enormous number of things sync up spontaneously. His research covered a wide range of phenomena, from sleep patterns to heart rhythms to the synchronous pulse of Asian fireflies. And now, in his new book, Sync, he’s drawn all these strands (and many others) together in a way that has the shock of the new. Even though we may see the moon every night (perhaps not realizing it’s an example of sync) it’s hard not to be surprised at the number of things around us—and in us—that must (or must not) sync up for things to go right.