Pseudoscience is theory or speculation which has the trappings and rhetoric of science, and is presented as science, but does not follow the scientific method. Pseudoscientific theories are typically not falsifiable, and their purveyors show unwillingness to allow outsiders to observe, test, or replicate their findings.
The most cited element of scientific theory is falsifiability, established by philosopher Karl Popper. A falsifiable theory makes predictions, which can be used to test it. Thus, the theory of gravity is falsifiable, as it predicts that an apple held above the ground and released will fall. If it fails the test, it is discarded, and if it passes, it is not proven, but better supported.
The term pseudoscience is also often applied to theories which are falsifiable and have in fact been falsified and discarded. Phrenology predicted that psychological traits could be inferred from the shape of the skull, but no actual link was ever found. Various astrological theories predicted regular planetary effects on the earth and human health, but testing those failed.
The existence of God is thus not considered scientific theory, as no test can possibly be conceived which categorically disproves His existence. The same is the case for string theory, which is a mathematical unification of scientific theory, but makes no predictions. Such things fall outside the realm of science.
The label pseudoscientist is commonly given to cranks and charlatans, who claim abnormal powers and abilities, but refuse to demonstrate them in objective, controlled, monitored environments. For example, Uri Geller claims to be able to bend spoons without applying pressure to them, but he cannot do it in front of scientists or journalists. Countless conjurers and mediums claim to be able to speak with the dead, but controlled circumstances revealed that they were really only conducting clever psychological tricks.
Spiritual, metaphysical, philosophical, and religious beliefs are occasionally confused with pseudoscience, as there is often an overlap in rhetoric, and charlatans blur the lines between scientific and philosophical ideas of knowledge.
Despite evidence and logic against pseudoscientific theories, many remain popular among the public. Homeopathic medicine is available at most health-food stores and is a multi-billion dollar business, and 31% of Americans believe in astrology.
There is no definitive or objective definition of the term "pseudoscience", so it is also sometimes used as a pejorative term. Pseudoscientists consider their work to be real science rather than pseudoscience. In addition to this, oftentimes theories which conflict with their theories are considered by them to be pseudoscience. Theories which are considered to be psuedoscience by the public may transition into being considered science if substantial evidence is found to support the theory. For example, before the theory of plate tectonics was fully developed, it was considered pseudoscience, whilst its predecessors, theories which involved an expanding Earth, are now considered pseudoscience.
Examples of pseudosciences
- Animal magnetism
- Big Bang astronomy
- Much of the work of Richard Dawkins (see: Richard Dawkins and pseudoscience)
- Biodynamic agriculture
- Evolutionary biology 
- Free Energy
- Global warming
- Much of the work of Stephen Hawking
- Much of the work of PZ Myers
- Old earth geology
- Orgone therapy
- Population control
- Self-projection as God (also known as vicarious autotheism)
- Much of the work of Carl Sagan
Refutations of Pseudoscience
- Counterexamples to Evolution
- Counterexamples to Global Warming
- Counterexamples to an Ice Age
- Counterexamples to an Old Earth
- Counterexamples to Relativity
- Counterexamples to String Theory
- Stanford Encyclopedia of Philosophy, see in particular sections 2 and 3
- http://www.youtube.com/watch?v=N3vGGf-ZIkc Uri Geller debunked by James Randi and Johnny Carson
- R. G. A. Dolby, Uncertain Knowledge: An Image of Science for a Changing World, Published by Cambridge University Press, 2002. ISBN 0521892627, 9780521892629, 380 pages. Page 191
- Brian Stableford, Science Fact and Science Fiction, Published by CRC Press, 2006, ISBN 0415974607, 9780415974608, 729 pages. Page 410