the Flat Earth Hypothesis because it's only a theory if it's commonly accepted by the general public while a hypothesis can be an observation made by a small group of theory.
Uhhh, this again? I know it's a semantics debate, but I'll give it another go. There is more than one definition for the word "theory" and flat-earth theory qualifies for the scientific definition as it is a body of knowledge that describes a phenomenon. Also, your definition is patently wrong. I recommend a dictionary.
Now, others will tell you that it's a zetetic theory. I'm fine with that, it's their prerogative to label what is or isn't a zetetic theory and we can't argue with that. I however, reject the zetetic ideals, but regardless of this, flat-earth theory is both a theory in the normal use and zetetic use of the word.