A general definition of pseudoscience is "any body of knowledge purporting to be factual and scientific, but which has failed to be validated in accordance with the scientific method".
Pseudoscience is distinguished from protoscience in its deliberate deviation from the accepted Scientific Method. Protoscience is science regarded to be in accordance with the method, but which has yet to be properly tested and either supported or refuted. Such fields as acupuncture and lucid dreaming may perhaps be best categorized as protosciences, pending more evidence and theoretical underpinning.
Typically, works of pseudoscience fail to meet the criteria laid down by the scientific method in one or more of the following ways:
- by asserting premises (claims without supporting evidence) as factual evidence,
- by asserting claims in contradiction of recorded evidence,
- by failing to provide a experimental framework for reproducible results, or
- by violating Occam's Razor (the principle of choosing the simplest explanation when multiple viable explanations are possible).
Pseudoscience is distinguished from Revelation, Theology or Spirituality in that it claims to offer insight into the physical world by scientific means (i.e., means in accordance with the scientific method]]). Systems of thought that rely upon "divine" or "inspired" knowledge are not considered pseudoscience if they do not claim to be scientific per se. A classic example of a work generally regarded as pseudoscientific is Dianetics: The Modern Science of Mental Health by L. Ron Hubbard, founder of the Church of Scientology.
The motivations for the promotion of pseudoscience range from simple naivety about the methodological rigour of the scientific method, to deliberate deception for financial exploitation (e.g., Psychic surgery).
Examples of fields of knowledge that can be considered pseudoscientific include astrology, numerology, cartomancy, and many other fortune-telling methods; dowsing, channeling, and other psychic claims; applied kinesiology, iridology and other non-scientific medical practices. There have in many cases been rigorous scientific studies conducted in these areas which have failed to substantiate many of the claims. Pseudoscientific medical practices often become quite popular, in part because they often work well due to the placebo effect. Many pseudosciences are associated with the New Age movement and there is a tendency to associate all practices of the "New Age" with pseudoscience.
-  The Skeptics Dictionary.
Although primarily the work of a single individual and considerably negative in its bias, this work is nonetheless a well-researched and cross-referenced presentation of pseudoscientific and related subjects.
-  James Randi Educational Foundation.
The JREF organization investigates lots of claims and attempts to test them in controlled experimental conditions. No luck yet finding any evidence of anything not explainable by real science. Lots of good information on the website.