Skepticism (US) or scepticism (UK) is a questioning attitude or doubt toward knowledge claims that are seen as mere belief or dogma. For example, if a person is skeptical about claims made by their government about an ongoing war then the person doubts that these claims are accurate. In such cases, skeptics normally recommend not disbelief but suspension of belief, i.e. maintaining a neutral attitude that neither affirms nor denies the claim. This attitude is often motivated by the impression that the available evidence is insufficient to support the claim. Formally, skepticism is a topic of interest in philosophy, particularly epistemology.

More informally, skepticism as an expression of questioning or doubt can be applied to any topic, such as politics, religion, or pseudoscience. It is often applied within restricted domains, such as morality (moral skepticism), atheism (skepticism about the existence of God), or the supernatural. Some theorists distinguish "good" or moderate skepticism, which seeks strong evidence before accepting a position, from "bad" or radical skepticism, which wants to suspend judgment indefinitely.

Philosophical skepticism is one important form of skepticism. It rejects knowledge claims that seem certain from the perspective of common sense. Radical forms of philosophical skepticism deny that "knowledge or rational belief is possible" and urge us to suspend judgment on many or all controversial matters. More moderate forms claim only that nothing can be known with certainty, or that we can know little or nothing about nonempirical matters, such as whether God exists, whether human beings have free will, or whether there is an afterlife. In ancient philosophy, skepticism was understood as a way of life associated with inner peace.

Skepticism has been responsible for many important developments in science and philosophy. It has also inspired several contemporary social movements. Religious skepticism advocates for doubt concerning basic religious principles, such as immortality, providence, and revelation. Scientific skepticism advocates for testing beliefs for reliability, by subjecting them to systematic investigation using the scientific method, to discover empirical evidence for them.

Skepticism, also spelled scepticism (from the Greek σκέπτομαι skeptomai, to search, to think about or look for), refers to a doubting attitude toward knowledge claims. So if a person is skeptical of their government's claims about an ongoing war then the person has doubts that these claims are true. Or being skeptical that one's favorite hockey team will win the championship means that one is uncertain about the strength of their performance. Skepticism about a claim implies that one does not believe the claim to be true. But it does not automatically follow that one should believe that the claim is false either. Instead, skeptics usually recommend a neutral attitude: beliefs about this matter should be suspended. In this regard, skepticism about a claim can be defined as the thesis that "the only justified attitude with respect to [this claim] is suspension of judgment". It is often motivated by the impression that one cannot be certain about it. This is especially relevant when there is significant expert disagreement. Skepticism is usually restricted to a claim or a field of inquiry. So religious and moral skeptics have a doubtful attitude about religious and moral doctrines. But some forms of philosophical skepticism, are wider in that they reject any form of knowledge.

Some definitions, often inspired by ancient philosophy, see skepticism not just as an attitude but as a way of life. This is based on the idea that maintaining the skeptical attitude of doubt toward most concerns in life is superior to living in dogmatic certainty, for example because such a skeptic has more happiness and peace of mind or because it is morally better. In contemporary philosophy, on the other hand, skepticism is often understood neither as an attitude nor as a way of life but as a thesis: the thesis that knowledge does not exist.

Skepticism is related to various terms. It is sometimes equated with agnosticism and relativism. However, there are slight differences in meaning. Agnosticism is often understood more narrowly as skepticism about religious questions, in particular, about the Christian doctrine. Relativism does not deny the existence of knowledge or truth but holds that they are relative to a person and differ from person to person, for example, because they follow different cognitive norms. The opposite of skepticism is dogmatism, which implies an attitude of certainty in the form of an unquestioning belief. A similar contrast is often drawn in relation to blind faith and credulity.

Various types of skepticism have been discussed in the academic literature. Skepticism is usually restricted to knowledge claims on one particular subject, which is why its different forms can be distinguished based on the subject. For example, religious skeptics distrust religious doctrines and moral skeptics raise doubts about accepting various moral requirements and customs. Skepticism can also be applied to knowledge in general. However, this attitude is usually only found in some forms of philosophical skepticism. A closely related classification distinguishes based on the source of knowledge, such as skepticism about perception, memory, or intuition. A further distinction is based on the degree of the skeptical attitude. The strongest forms assert that there is no knowledge at all or that knowledge is impossible. Weaker forms merely state that one can never be absolutely certain.