Superstition: Belief in what believers in it could know is not so.
What is superstition depends on what believers in it could know, if they had taken the trouble: If there are no good - rational, empirical - arguments against the belief, and emotional, cultural, religious, political, moral or practical reasons for it, one cannot seriously blame the believers in it.
But if there is good evidence against the belief, even if the believer in the superstition is disinclined to get informed about it, and the believer could reasonably know of this evidence (had he tried to find out), then his belief is fairly called superstitious, especially if such a belief, if acted upon, may harm people.
In fact, there is a lot of superstition, concerning many subjects, varying from religion and politics to medicine - healthcare - and psychology and related subjects.
Also, what one regards as superstition - a term normally used with some disapprobation - as contrasted with insufficiently well-founded belief may to some extent depend on taste, but there are many superstitions (in witches, in the potency-improving powers of rhino-horns, in miracle-cures by self-styled mediums etc.) that in this day and age are plainly superstituous, and deserve to be called superstitions.
Also, myself being an atheist, and being well-informed as regards the scientific evidence, I hold that the Dutch 19th Century writer Multatuli, a severe critic of religion of great sarcastic power, was right in his saying that
"Religion is superstition"
(And please note, if you disagree, that I am not talking about some sophisticated belief in the possibility that there may be some deity, albeit one humans know and understand very little about, but about the foundational texts of religions, such as the Bible and the Koran: These cannot be true as stated in these books, for which reason belief in their approximate truth must be classed as superstition.)