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An encyclopedia^[a] is a reference work or compendium providing summaries of knowledge, either general or special, in a particular field or discipline.^[1][2] Encyclopedias are divided into articles or entries that are arranged alphabetically by article name^[3] or by thematic categories, or else are hyperlinked and searchable.^[4] Encyclopedia entries are longer and more detailed than those in most dictionaries.^[3][5] Generally speaking, encyclopedia articles focus on factual information concerning the subject named in the article's title;^[5] this is unlike dictionary entries, which focus on linguistic information about words, such as their etymology, meaning, pronunciation, use, and grammatical forms.^{[5][6][7][8][9]}

Encyclopedias have existed for around 2,000 years^[10] and have evolved considerably during that time as regards language (written in a major international or a vernacular language), size (few or many volumes), intent (presentation of a global or a limited range of knowledge), cultural perspective (authoritative, ideological, didactic, utilitarian), authorship (qualifications, style), readership (education level, background, interests, capabilities), and the technologies available for their production and distribution (hand-written manuscripts, small or large print runs, Internet). As a valued source of reliable information compiled by experts, printed versions found a prominent place in libraries, schools and other educational institutions.^[11]

In the 21st century, the appearance of digital and open-source versions such as Wikipedia (together with the wiki website format) has vastly expanded the accessibility, authorship, readership, and variety of encyclopedia entries.^[12]