Would it be a hunch or a notion. Maybe a belief? It’s misconstured as proof but isn’t because it doesn’t possess the requirements to be proof
A "fallacy" is argument that might appear true, but is an error in reasoning, and will invalidate the argument. For example:
If you argue
Since the students have no questions concerning the topics discussed in class, the students are ready for a test.
You are making an error. This is the "Fallacy of Appeal to Ignorance" as the lack of evidence that the students are ignorant does not prove that they are not ignorant. An absence of evidence is not evidence of absence.
In contemporary usage, the word myth can mean "something which people accept and perpetuate as fact but which is not based on fact". The phrase urban legend has much the same meaning.
That the Inuit have more than 50 words for snow was thought to be a myth.