Pseudoscience is theory or speculation which has the trappings and rhetoric of science, and presented as science, but not generally accepted as valid by the scientific community. Theories are typically classed as pseudoscientific if they fail the scientific method. Pseudoscientific theories are typically no falsifiable, lack objectivity, and their purveyors show unwillingness to allow neutral 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 of pseudoscience is commonly given to 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 reveal that they are 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.
Examples of pseudosciences
- Animal magnetism
- Chiropractic (Note that the term now refers to two distinct practices. The original idea, that subluxations of the spine cause all illness, has been utterly rejected)
- Evolutionary biology 
- Orgone therapy