Fringe science is a phrase used to describe scientific inquiry in an established field that departs significantly from mainstream or orthodox theories. Fringe science can be a field of inquiry which is not yet considered a real "science" (known as a protoscience), but which nevertheless bears some resemblance to the norms of the scientific method. Fringe science is, by definition, at the fringes of an already accepted discipline. Based upon the merits of the hypothesis and methods the inquiry, specific instances of fringe science may or may not come to be included in the canon of actual science.

Fringe science can be distinguished from some similar sounding, but pejorative in nature, categories as follows:

• Pseudoscience - Pseudoscience is notoriously lax in rigorous application of the scientific method. Reproducibility is typically a problem. This is not so in fringe science.
• Junk science - Junk science is used to describe agenda-driven research that ignores certain standard methodologies and practices in an attempt to secure a given result from an experiment. Fringe science, as in standard methodology, proceeds from theory to conclusion with no attempt to direct or coax the result.
• Bad science - Bad science might more properly be labeled "poor science" in that it is typically characterized by substandard or "sloppy" methodology. Fringe science maintains the normal standards of methodology.

Fringe science is, simply, potentially real scientific inquiry that is on the edges of mainstream and widely-accepted theories. Fringe science is seen by most scientists as unlikely but not irrational; a number of today's most widely-held theories had their origins as fringe science.

As with all categories, disagreement is widespread regarding what ideas are legitimate fringe science, and what ideas belong to the other categories listed above. Traditionally, the term "fringe science" is generally used to describe unusual or fantastic theories that have their basis in some established scientific principle, and which are advocated by a published (or somehow recognized) mainstream scientist.