Stories about relationships that are based on accidental coincidence.
A black cat runs under a ladder and you have some bad luck. Perhaps you never notice the times something like that happened and good luck came your way.
The second thing that comes to mind when I hear the world superstition is the irrational reactions of supposedly well-meaning, otherwise rational people have when talking about religion, spirituality, miracles and the like.
Religion is spirituality corrupted by self-concern (ego). It's not all bad, but it has lost the spark of Truth from which it had been born.
Some people reject spirituality and tales of miracles without good reason. They claim it's superstition (accidental coincidence, instead of cause-and-effect coincidence).
Example: A man feels a rush of inspiration, finds a loving peace that makes them feel invulnerable and unconcerned about what others might do to their body, and asks for something impossible. The next instant, they are walking on water, or parting water, or something else equally incredible. There's nothing accidental about it, yet "reasoning" skeptics see only physical law and some "trick" they cannot yet explain. This is the worst kind of superstition -- the kind that thinks it is being reasonable, but is flushing all reason down the toilet in favor of the status quo -- the safe realm where miracles are "impossible."