Wikipedia is a multilingual free online encyclopedia written and maintained by a community of volunteers, known as Wikipedians, through open collaboration and using a wiki-based editing system called MediaWiki. Wikipedia is the largest and most-read reference work in history. It is consistently one of the 10 most popular websites ranked by Similarweb and formerly Alexa; as of 2023, Wikipedia was ranked the 5th most popular site in the world according to Semrush. It is hosted by the Wikimedia Foundation, an American non-profit organization funded mainly through donations.

Wikipedia was launched by Jimmy Wales and Larry Sanger on January 15, 2001. Sanger coined its name as a blend of wiki and encyclopedia. Wales was influenced by the "spontaneous order" ideas associated with Friedrich Hayek and the Austrian School of economics after being exposed to these ideas by the libertarian economist Mark Thornton. Initially available only in English, versions in other languages were quickly developed. Its combined editions comprise more than 60 million articles, attracting around 2 billion unique device visits per month and more than 15 million edits per month (about 5.7 edits per second on average) as of January 2023. In 2006, Time magazine stated that the policy of allowing anyone to edit had made Wikipedia the "biggest (and perhaps best) encyclopedia in the world".

Wikipedia has been praised for its enablement of the democratization of knowledge, extent of coverage, unique structure, culture, and reduced degree of commercial bias. It has been criticized for exhibiting systemic bias, particularly gender bias against women and ideological bias. While the reliability of Wikipedia was frequently criticized in the 2000s, it has improved over time, receiving greater praise in the late 2010s and early 2020s. The website's coverage of controversial topics such as American politics and major events like the COVID-19 pandemic and the Russian invasion of Ukraine has received substantial media attention. It has been censored by world governments, ranging from specific pages to the entire site. On 3 April 2018, Facebook and YouTube announced that they would help users detect fake news by suggesting fact-checking links to related Wikipedia articles. Articles on breaking news are often accessed as a source of frequently updated information about those events.