From the tag page: 

 A fringe theory is an idea or a collection of ideas that departs significantly from the prevailing or mainstream view. It can include work done to the appropriate level of scholarship in a field of study but only supported by a minority of practitioners, to more dubious work. Examples of the latter include pseudoscience (ideas that purport to be scientific theories but have little or no scientific support), conspiracy theories, unproven claims about alternative medicine, pseudohistory and so forth.