Freethought (sometimes spelled free thought) is an epistemological viewpoint which holds that beliefs should not be formed on the basis of authority, tradition, revelation, or dogma, and that beliefs should instead be reached by other methods such as logic, reason, and empirical observation. According to the Oxford English Dictionary, a freethinker is "a person who forms their own ideas and opinions rather than accepting those of other people, especially in religious teaching." In some contemporary thought in particular, free thought is strongly tied with rejection of traditional social or religious belief systems. The cognitive application of free thought is known as "freethinking", and practitioners of free thought are known as "freethinkers". Modern freethinkers consider free thought to be a natural freedom from all negative and illusive thoughts acquired from society.

The term first came into use in the 17th century in order to refer to people who inquired into the basis of traditional beliefs which were often accepted unquestioningly. Today, freethinking is most closely linked with deism, secularism, humanism, anti-clericalism, and religious critique. The Oxford English Dictionary defines freethinking as, "The free exercise of reason in matters of religious belief, unrestrained by deference to authority; the adoption of the principles of a free-thinker." Freethinkers hold that knowledge should be grounded in facts, scientific inquiry, and logic. The skeptical application of science implies freedom from the intellectually limiting effects of confirmation bias, cognitive bias, conventional wisdom, popular culture, urban myth, prejudice, or sectarianism.