Pseudoscience consists of statements, beliefs, or practices that claim to be both scientific and factual but are incompatible with the scientific method. 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; absence of systematic practices when developing hypotheses; and continued adherence long after the pseudoscientific hypotheses have been experimentally discredited.

The demarcation between science and pseudoscience has scientific, philosophical, and political implications. Philosophers debate the nature of science and the general criteria for drawing the line between scientific theories and pseudoscientific beliefs, but there is widespread agreement "that creationism, astrology, homeopathy, Kirlian photography, dowsing, ufology, ancient astronaut theory, Holocaust denialism, Velikovskian catastrophism, and climate change denialism are pseudosciences." There are implications for health care, the use of expert testimony, and weighing environmental policies. Addressing pseudoscience is part of science education and developing scientific literacy.

Pseudoscience can have dangerous effects. For example, pseudoscientific anti-vaccine activism and promotion of homeopathic remedies as alternative disease treatments can result in people forgoing important medical treatments with demonstrable health benefits, leading to deaths and ill-health. Furthermore, people who refuse legitimate medical treatments for contagious diseases may put others at risk. Pseudoscientific theories about racial and ethnic classifications have led to racism and genocide.

The term pseudoscience is often considered pejorative, particularly by purveyors of it, because it suggests something is being presented as science inaccurately or even deceptively. Therefore, those practicing or advocating pseudoscience frequently dispute the characterization.