Dividing the category again, unscientific claims are a subset of the large category of non-scientific claims.This category specifically includes all matters that are directly opposed to good science.Among the earliest uses of "pseudo-science" was in an 1844 article in the Northern Journal of Medicine, issue 387: That opposite kind of innovation which pronounces what has been recognized as a branch of science, to have been a pseudo-science, composed merely of so-called facts, connected together by misapprehensions under the disguise of principles.

Pseudoscience is often characterized by contradictory, exaggerated or unfalsifiable claims; reliance on confirmation bias rather than rigorous attempts at refutation; lack of openness to evaluation by other experts; and absence of systematic practices when developing theories, and continued adherence long after they have been experimentally discredited.

The term pseudoscience is considered pejorative Distinguishing scientific facts and theories from pseudoscientific beliefs, such as those found in astrology, alchemy, medical quackery, occult beliefs, and creation science, is part of science education and scientific literacy.

Thus pseudoscience is a subset of un-science, and un-science, in turn, is subset of non-science.

Pseudoscience is differentiated from science because – although it claims to be science – pseudoscience does not adhere to accepted scientific standards, such as the scientific method, falsifiability of claims, and Mertonian norms.

Larry Laudan has suggested pseudoscience has no scientific meaning and is mostly used to describe our emotions: "If we would stand up and be counted on the side of reason, we ought to drop terms like 'pseudo-science' and 'unscientific' from our vocabulary; they are just hollow phrases which do only emotive work for us".

Likewise, Richard Mc Nally states, "The term 'pseudoscience' has become little more than an inflammatory buzzword for quickly dismissing one's opponents in media sound-bites" and "When therapeutic entrepreneurs make claims on behalf of their interventions, we should not waste our time trying to determine whether their interventions qualify as pseudoscientific. Ravetz "pseudo-science may be defined as one where the uncertainty of its inputs must be suppressed, lest they render its outputs totally indeterminate".

Apart from theoretical issues, pseudoscience can cause severe negative consequences in the real world.

For example, antivaccine activists present pseudoscientific studies that falsely claim that the science on the safety of vaccines is uncertain, which has led to many deaths.

A typical 19th-century phrenology chart: During the 1820s, phrenologists claimed the mind was located in areas of the brain, and were attacked for doubting that mind came from the nonmaterial soul.