The oppisite of a superstition, a substition is the belief in something that is utterly true. Coined by Terry Pratchett in 'Making Money'.
When my mother told me not to pick at my scab or it wouldn't heal, I saw immediately that she was very substicious (believing greatly in a substition).
The evil entity that comes to women once a month, resulting in said female becoming the biggest bitch of the best.
Ed: Dude, did you see how she blew me off?
Bill: Yea, she's got the menstural monster.