Wikipedia is a free, online encyclopedia created through the collaborative efforts of a global community of volunteers. Launched in 2001, it has grown into the largest and most widely read reference work in history, available in over 300 languages. Hosted by the Wikimedia Foundation, a non-profit organization funded by reader donations, Wikipedia attracts billions of visits monthly and features millions of articles across its various language editions. While praised for democratizing knowledge and its extensive coverage, it has also faced criticism regarding systemic biases and occasional censorship by governments. Its reliability, initially questioned, has improved significantly over time.

Originating as a supplementary project to Nupedia, Wikipedia quickly adopted a wiki format for open editing. Its core policy of maintaining a "neutral point of view" was established early on. Despite initial plans for commercialization, it transitioned to a non-profit model and secured its domain as .org. The platform has experienced periods of rapid growth followed by a slowing rate, leading to discussions about editor retention and the nature of content creation. Wikipedia has also inspired numerous sister projects, such as Wiktionary and Wikimedia Commons, expanding its scope of collaborative knowledge sharing.

Wikipedia began in 2001 as a complementary initiative to Nupedia, an expert-written, formally reviewed encyclopedia. Nupedia, owned by the web portal company Bomis, was spearheaded by Jimmy Wales and Larry Sanger. Sanger proposed the creation of a wiki as a 'feeder' project for Nupedia, aiming for a publicly editable encyclopedia. This concept, influenced by ideas of spontaneous order and the Austrian School of economics, led to the launch of Wikipedia. Initially a single English edition, it quickly established a "neutral point of view" policy. Early contributors were drawn from Nupedia and online communities. Language editions soon followed, and Nupedia was eventually absorbed into Wikipedia after its servers were retired in 2003.

Wikipedia experienced exponential growth in its early years, surpassing historical encyclopedias in size. However, growth rates, particularly for the English edition, have fluctuated. Concerns about declining editorship emerged in the late 2000s and early 2010s, with various analyses suggesting reasons ranging from complex editing rules to the natural saturation of easily created articles. Page view numbers also saw shifts, with external factors like search engine knowledge graphs potentially influencing user behavior. Despite these challenges, Wikipedia has maintained a significant global presence, consistently ranking among the world's most visited websites. Its reliability has been a subject of discussion, with notable improvements recognized from the late 2010s onwards.

Beyond its core encyclopedia function, Wikipedia has fostered a suite of related 'sister projects' under the umbrella of the Wikimedia Foundation. These include Wiktionary for dictionaries, Wikiquote for quotations, Wikibooks for textbooks, Wikimedia Commons for multimedia, Wikinews for journalism, and Wikiversity for learning materials. Projects like Wikispecies and Wikivoyage have further expanded the scope of collaborative knowledge. Wikipedia's impact extends to its role in the democratization of knowledge, though it has also faced governmental censorship and criticism for inherent biases. Its open-source, community-driven model has been both lauded and scrutinized, reflecting its complex position in the digital information landscape.