Pseudoscience is defined as a body of unproven and/or uncorroborated knowledge, customs, traditions, methodologies, beliefs, or practices that are alleged to be scientific or made to appear scientific, but does not adhere to the scientific method, For example, Hewitt et al. Conceptual Physical Science Addison Wesley; 3 edition (July 18, 2003) ISBN 0-321-05173-4, Bennett et al. The Cosmic Perspective 3e Addison Wesley; 3 edition (July 25, 2003) ISBN 0-8053-8738-2 lacks supporting evidence or plausibility, or otherwise lacks scientific status. The term comes from the Greek root pseudo- (false or pretending) and "science" (from Latin scientia, meaning "knowledge"). An early recorded use was in 1843 by French physiologist François Magendie, Magendie, F (1843) An Elementary Treatise on Human Physiology. 5th Ed. Tr. John Revere. New York: Harper, p 150. Magendie refers to phrenology as "a pseudo-science of the present day" (note the hyphen). who is considered a pioneer in experimental physiology.

As it is taught in certain introductory science classes, pseudoscience is any subject that appears superficially to be scientific or whose proponents state is scientific but nevertheless contravenes the testability requirement, or substantially deviates from other fundamental aspects of the scientific method. Professor Paul DeHart Hurd argued that a large part of gaining scientific literacy is "being able to distinguish science from pseudo-science such as astrology, quackery, the occult, and superstition". Certain introductory survey classes in science take careful pains to delineate the objections scientists and skeptics have to practices that make direct claims contradicted by the scientific discipline in question.

Beyond the initial introductory analyses offered in science classes, there is some epistemological disagreement about the extent to which it is possible to distinguish "science" from "pseudoscience" in a reliable and objective way. The term itself has negative connotations, because it is used to indicate that subjects so labeled are inaccurately or deceptively portrayed as science. Accordingly, those labeled as practicing or advocating a "pseudoscience" normally reject this classification.

Pseudosciences have been characterised by the use of vague, exaggerated or untestable claims, over-reliance on confirmation rather than refutation, lack of openness to testing by other experts, and a lack of progress in theory development.