Wikipedia
Wikipedia is a free, volunteer-written online encyclopedia operated by the nonprofit Wikimedia Foundation. It began with its first edit on 15 January 2001, grew out of the expert-reviewed Nupedia project, and became the world's largest reference website, with more than 68 million articles across more than 300 language editions and over a billion visitors each month. Its content is governed by five pillars and by policies such as a neutral point of view and verifiability, and it is licensed for reuse under Creative Commons BY-SA. Studies — from Nature's 2005 comparison with Encyclopædia Britannica to peer-reviewed research on coverage gaps — have both tested its accuracy and documented where it falls short; in the 2020s the Wikimedia Foundation reports declining human pageviews attributed to generative AI while describing Wikipedia's human-curated knowledge as more valuable than ever.