Wikipedia:What is fringe?
What is a fringe theory?
One can think of fringe theories as ranging from theories that almost qualify as alternative mainstream theories down to things that have just barely too many scientific chops to be called pseudoscience.
As an analogy, consider the shadow of a large object illuminated by a single extended light source. One part of its shadow is called the penumbra. The penumbra is the region where the light from the source is partly blocked; it runs from full light at one side to full shadow at the other. Another part of the shadow is called the umbra. The umbra is the region where the light is completely blocked. In this analogy, the penumbra corresponds to fringe science, and the umbra corresponds to pseudoscience, and the places where the light is not blocked correspond to mainstream science.
One consequence of this view is that the borderline between the upper end of fringe science and the lower end of alternative mainstream science is hard to distinguish. Similarly, the border between the lower end of fringe science and the upper end of pseudoscience is hard to distinguish.
Some fringe theories were taken seriously as alternative mainstream theories for at least a little while, and were then rejected. Plasma cosmology would be an example.
The original version of this was posted here