An encyclopedia (also spelled encyclopaedia or encyclopædia) is a comprehensive written compendium that holds information from either all branches of knowledge or a particular branch of knowledge. Encyclopedias are divided into articles with one article on each subject covered. The articles on subjects in an encyclopedia are usually accessed alphabetically by article name and can be contained in one volume or many volumes, depending on the amount of material included.

Indeed, the purpose of an encyclopedia is to collect knowledge disseminated around the globe; to set forth its general system to the men with whom we live, and transmit it to those who will come after us, so that the work of preceding centuries will not become useless to the centuries to come; and so that our offspring, becoming better instructed, will at the same time become more virtuous and happy, and that we should not die without having rendered a service to the human race in the future years to come.

—Diderot

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